Citus 12.1 documentation update checklist

[hanefi]

• [x] Ensure git cherry-picks up to date on master branch
• [x] Bump version
  • ~:vimgrep '\(\<11\.\?3\>\)\|\(113_\)' **/*.rst *.py~
  • Both Citus 12.0 -> 12.1 and PG 15 -> 16.
• [x] Update installation
• [x] Update our upgrade steps
• [x] Update citusdata.com/download links.
• [x] scan for any other udf and guc changes (in Community and Enterprise)
  • git diff origin/release-11.3 origin/release-12.0 -- src/backend/distributed/shared_library_init.c
  • git diff origin/release-11.3 origin/release-12.0 -- src/backend/distributed/sql/udfs/
  • Also check https://github.com/citusdata/citus/pull/7049
• [x] Verify there are no Sphinx warnings in local build
  • sphinx-build -b html -a -n -W . _build
  • (Remove _build directory and try fresh too)
• [x] Create new 12.0 release branch (v12.1)
• [x] Set as default ver in RTD (advanced settings tab)
• [x] Rebuild previous version so that canonical link gets updated