Add subscripts to issue templates

[Copilot]

This PR adds helpful subscripts to all issue templates to encourage proper issue prioritization and engagement practices.

Changes Made

Markdown-based templates

Added the following subscript to the bottom of:

• discussion.md
• feature_request.md
• request_for_instrumentation.md

<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>

YAML-based issue form

Added a dropdown element to bug_report.yaml with the subscript:

  - type: dropdown
    attributes:
      label: Tip
      description: This element is static, used to render a helpful sub-heading for end-users and community members to help prioritize issues. Please leave as is.
      options:
        - <sub>[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>
      default: 0

Impact

These subscripts will help:

• Guide users to use 👍 reactions instead of +1 comments for prioritization
• Encourage meaningful context in comments rather than noise
• Direct users to community participation guidelines
• Improve issue triage efficiency

The changes are minimal and surgical - only 13 lines added across 4 files with no deletions.

Fixes #44.

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