docs(site redesign): Overhaul Docs Site

[maggiehays]

There's a lot packed in this PR; here are the main changes:

New landing page for datahubproject.io

New landing page for datahubproject.io/docs

New sidebar organization

Compiling open-source and managed DataHub docs in a single place

• Sidebar will show "cloud" icon on Managed-DataHub-only resources
• New page template to call out features that are Managed-DataHub-only vs. available in OSS
  

The following is out of scope for this PR & will be addressed in fast-follow work:

• add consistency to "tutorial" vs. "usage guide" vs. "feature guide" naming conventions
• migrate existing Managed DataHub docs to the unified docs site
• write up all 31 feature guide/usage guide docs in the /docs/wip/ directory
  • once complete, migrate them to main docs directory
• create missing guides:
  • Entity health check status
  • Column level lineage
  • Groups
  • Entity usage statistics & Profiling

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

[github-actions]

Unit Test Results (build & test)

504 tests  ±0   504 ✔️ ±0   9m 30s ⏱️ +15s
117 suites ±0       0 💤 ±0 
117 files   ±0       0 ❌ ±0 

Results for commit bc0c503. ± Comparison against base commit e448bb8.

♻️ This comment has been updated with latest results.