docs: adds contributing guide and issue templates (#121)

Author: sufyankhanrao

.github/CONTRIBUTING.md

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+# Contributing to apimatic/core-lib-java
+
+Thank you for your interest in contributing! 🎉 Your contributions help make this project better. We value open-source contributions to this library, please take a few minutes to review this guide before you start.
+
+---
+
+## 💡 How to Contribute
+
+### 📌 1. Reporting Issues
+
+- Search existing issues before opening a new one.
+- Use a **descriptive title** and provide clear steps to reproduce.
+- Include relevant logs, screenshots, or error messages.
+
+### 🔧 2. Making Changes
+
+#### Step 1: Create a Branch
+
+- Create a new branch from `master`:
+  ```sh
+  git checkout -b your-feature-name
+  ```
+
+#### Step 2: Make Your Changes
+
+- Follow the project's **coding standards**.
+- Add **unit tests** if applicable.
+- Ensure your changes **do not break existing functionality**.
+
+#### Step 3: Commit Changes
+
+- Use clear and descriptive commit messages:
+  ```sh
+  git commit -m "feat: Add feature description"
+  ```
+
+#### Step 4: Push & Open a PR
+
+- Push your branch:
+  ```sh
+  git push origin your-feature-name
+  ```
+- Open a **Pull Request (PR)** on GitHub:
+  - Provide a clear **description** of the changes.
+  - Mention related **issue numbers**.
+  - Request a **review** from maintainers.
+  - Make sure your changes clear all the **PR Checks**.
+
+---
+
+## 📜 License
+
+By contributing, you agree that your contributions will be licensed under the project's [LICENSE](LICENSE).
+
+---
+
+💬 **Questions?**  
+If you need help, feel free to open issues, we will be happy to help.
+
+Happy Coding! 🚀

.github/ISSUE_TEMPLATE/bug-report.md

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+name: 🐞 Bug
+description: Report a bug or an issue you've found
+title: "[Bug] <title>"
+labels: ["bug"]
+body:
+  - type: markdown
+    attributes:
+      value: |
+        Thanks for taking the time to fill out this bug report!
+  - type: checkboxes
+    attributes:
+      label: Is this a new bug?
+      description: >
+        In other words: Is this an error, flaw, failure or fault? Please search issues to see if someone has already reported the bug you encountered.
+      options:
+        - label: I believe this is a new bug
+          required: true
+        - label: I have searched the existing issues, and I could not find an existing issue for this bug
+          required: true
+
+  - type: textarea
+    attributes:
+      label: Current Behavior
+      description: A concise description of what you're experiencing.
+    validations:
+      required: true
+
+  - type: textarea
+    attributes:
+      label: Expected Behavior
+      description: A concise description of what you expected to happen.
+    validations:
+      required: true
+
+  - type: textarea
+    attributes:
+      label: Steps To Reproduce
+      description: Steps to reproduce the behavior.
+      placeholder: |
+        1. In this environment...
+        2. With this config...
+        3. Run '...'
+        4. See error...
+    validations:
+      required: true
+
+  - type: textarea
+    attributes:
+      label: Environment
+      description: |
+        examples:
+          - **OS**: Ubuntu 20.04
+          - **Language version**: Java 8 (`java --version`)
+          - **SDK Name**: PayPal (If you are using this library as a dependency, please name the parent SDK)
+      value: |
+        - **OS**:
+        - **Language version**:
+        - **SDK Name**:
+      render: markdown
+    validations:
+      required: true
+
+  - type: textarea
+    id: logs
+    attributes:
+      label: Relevant log output
+      description: |
+        If applicable, log output to help explain your problem.
+      render: shell
+    validations:
+      required: false
+
+  - type: textarea
+    attributes:
+      label: Additional Context
+      description: |
+        Links? References? Anything that will give us more context about the issue you are encountering!
+    validations:
+      required: false

.github/ISSUE_TEMPLATE/bug-report.yml

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+name: ✨ Feature
+description: Propose a straightforward extension
+title: "[Feature] <title>"
+labels: ["enhancement"]
+body:
+  - type: markdown
+    attributes:
+      value: |
+        Thanks for taking the time to fill out this feature request!
+  - type: checkboxes
+    attributes:
+      label: Is this your first time submitting a feature request?
+      description: >
+        We want to make sure that features are distinct and discoverable,
+        so that other members of the community can find them and offer their thoughts.
+
+        Issues are the right place to request straightforward extensions of existing functionality.
+      options:
+        - label: I have searched the existing issues, and I could not find an existing issue for this feature
+          required: true
+        - label: I am requesting an extension of the existing functionality
+  - type: textarea
+    attributes:
+      label: Describe the feature
+      description: A clear and concise description of what you want to happen.
+    validations:
+      required: true
+  - type: textarea
+    attributes:
+      label: Describe alternatives you've considered
+      description: |
+        A clear and concise description of any alternative solutions or features you've considered.
+    validations:
+      required: false
+  - type: textarea
+    attributes:
+      label: Who will this benefit?
+      description: |
+        What kind of use case will this feature be useful for? Please be specific and provide examples, this will help us prioritize properly.
+    validations:
+      required: false
+  - type: input
+    attributes:
+      label: Are you interested in contributing this feature?
+      description: Let us know if you want to write some code, and how we can help.
+    validations:
+      required: false
+  - type: textarea
+    attributes:
+      label: Anything else?
+      description: |
+        Links? References? Anything that will give us more context about the feature you are suggesting!
+    validations:
+      required: false