feat(docs-website): support category links

[hsheth2]

This way we can get rid of all those "Overview" docs and just make them the category header.

I'd suggest reviewing this PR with "hide whitespace changes" enabled.

Checklist

• The PR conforms to DataHub's Contributing Guideline (particularly Commit Message Format)
• Links to related issues (if applicable)
• Tests for the changes have been added/updated (if applicable)
• Docs related to the changes have been added/updated (if applicable). If a new feature has been added a Usage Guide has been added for the same.
• For any breaking change/potential downtime/deprecation/big changes an entry has been made in Updating DataHub

[shirshanka]

LGTM!