docs: Update README with comprehensive documentation

[rhuss]

This PR updates the README.md file with comprehensive documentation covering the project's purpose, features, configuration (.deviate.yaml), how deviate sync works, a quickstart guide for GitHub Actions, and instructions for auto-merging with Mergify.

This PR was created via Cursor using gemini-2.5-pro with the following prompt

this projects allows for easy syncing an applying patches from an upstream repository. Unfortunately, the documentation is missing. Can you fix this by looking into the full project and source code ?

The documentation should contain a quickststart how to use this project with a GitHub workflow, how sync PRs are created and how they can be automerged by the mergify bot.

[openshift-ci]

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This pull-request has been approved by: rhuss
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