Add hashtag to headings

[lamATnginx]

Proposed changes

Closes https://github.com/nginxinc/docs-platform/issues/433

Video to demonstrate hovering shows # within its bounds + there is a # on the current anchor link:
https://github.com/user-attachments/assets/9f155f24-da60-40f1-b86d-eb5d5609c849

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING document
• If applicable, I have added tests that prove my fix is effective or that my feature works
• If applicable, I have checked that any relevant tests pass after adding my changes
• I have updated any relevant documentation (README.md and CHANGELOG.md)

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[lamATnginx]

Need suggestions on if the text and not just the # icon should be clickable. Similar to https://zed.dev/docs/#configure-zed

If so, the render-heading file would need to be refactored, potentially affecting old theme.

[nginx-jack]

@lamATnginx I think we'd want it to be clickable for sure - might need to data-mf="false" / data-mf="true the old and new stuff respectively 🤔 .

[lamATnginx]

Need a design pass but here is the updated video

Screen.Recording.2025-04-22.at.8.37.10.AM.mov

[lamATnginx]

Need a design pass but here is the updated video
Screen.Recording.2025-04-22.at.8.37.10.AM.mov

Heading color has been changed to black + removed hover underline.

[nginx-jack]

LGTM locally, nice one Lam!