Merge branch 'psanan/docs-upgrade-sphinx-4.2.0' into 'release'

Docs: upgrade to Sphinx 4.2.0

See merge request petsc/petsc!4475

Docs: upgrade to Sphinx 4.2.0

The functions overridden by our custom extension, visit_Text and
visit_literal_block, have not changed since 3.5.4.

Merge remote-tracking branch 'origin/release'

Merge branch 'psanan/docs-panels-team-table' into 'release'

Docs: Use sphinx-panels for team table

See merge request petsc/petsc!4409

Docs: Use sphinx-panels for team table

Use the sphinx-panels extension (which has CSS flexbox under the hood)
instead of custom CSS and auto-generation for team table.

Images are used in the normal

Docs: Use sphinx-panels for team table

Use the sphinx-panels extension (which has CSS flexbox under the hood)
instead of custom CSS and auto-generation for team table.

Images are used in the normal way for Sphinx (so should work with all
builders including html and dirhtml). Images are a mix of external
links and images in the PETSc images repo.

Uses hard-coded links to GitLab avatars, which were obtained with the
generate_team_table.py script.

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Merge remote-tracking branch 'origin/release'

Merge branch 'psanan/docs-upgrade-sphinx-3.5.4' into 'release'

Docs: upgrade to Sphinx 3.5.4

See merge request petsc/petsc!3863

Docs: upgrade to Sphinx 3.5.4

Change error on mismatched version to a warning, expecting most
builds and users to respect requirements.txt

Merge remote-tracking branch 'origin/release'

Merge branch 'psanan/docs-move-to-toplevel-doc' into 'release'

Docs: move src/docs/sphinx_docs/ to doc/

See merge request petsc/petsc!3866

Docs: move src/docs/sphinx_docs to doc/, without updating paths in the doc source