Add subscript to issue templates

[Copilot]

This PR adds a subscript to all issue templates in the repository to help users understand how to properly engage with issues and help maintainers prioritize them.

Changes Made

8 issue templates updated:

• 7 Markdown templates - Added markdown subscript to the bottom of each template
• 1 YAML template - Added dropdown element with the subscript text

Templates updated:

• auto-merge/js/.github/ISSUE_TEMPLATE/bug_report.yaml (YAML format)
• auto-merge/js/.github/ISSUE_TEMPLATE/feature_request.md (Markdown format)
• auto-merge/js/.github/ISSUE_TEMPLATE/request_for_instrumentation.md (Markdown format)
• auto-merge/js/.github/ISSUE_TEMPLATE/discussion.md (Markdown format)
• auto-merge/contrib/.github/ISSUE_TEMPLATE/bug_report.md (Markdown format)
• auto-merge/contrib/.github/ISSUE_TEMPLATE/feature_request.md (Markdown format)
• auto-merge/contrib/.github/ISSUE_TEMPLATE/plugin_request.md (Markdown format)
• auto-merge/contrib/.github/ISSUE_TEMPLATE/discussion.md (Markdown format)

Subscript Content

The subscript includes:

• Instructions to use 👍 reactions to help prioritize issues
• Guidance to provide useful context in comments rather than +1 or me too
• Link to community participation guidelines

For markdown templates, the subscript appears as:

<sub>**Tip**: [React](https://github.blog/news-insights/product-news/add-reactions-to-pull-requests-issues-and-comments/) with 👍 to help prioritize this issue. Please use comments to provide useful context, avoiding `+1` or `me too`, to help us triage it. Learn more [here](https://opentelemetry.io/community/end-user/issue-participation/).</sub>

For the YAML template, it's implemented as a dropdown element with the same content.

Validation

• All 8 templates confirmed to contain the required subscript
• YAML syntax validated successfully
• No existing template functionality disrupted

Fixes #17.

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