Add subscript to issue templates

[Copilot]

Added the required subscript to both markdown-based issue templates to help users understand how to effectively participate in issue triage and prioritization.

Changes Made

• Added subscript to .github/ISSUE_TEMPLATE/bug_report.md
• Added subscript to .github/ISSUE_TEMPLATE/feature_request.md

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 the OpenTelemetry community documentation for issue participation

Example

The subscript appears at the bottom of each template:

<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>

This will help improve the quality of issue discussions and make it easier for maintainers to prioritize work based on community feedback.

Fixes #21.

---

💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.