[StableOnly] Fix docs job

[StableOnly] Fix docs job

This is a combination of multiple commits from master:

  60a7b11d docs: pin mistune version due to breakage
  7544f8f5 requirements: Bump doc requirements
  0968130a Revert "docs: prevent build error by rolling back Sphinx version"
  65547c87 docs: prevent build error by rolling back Sphinx version
  bdb049c7 requirements: Update doc requirements

Though we don't usually change requirements on stable branches, this one
seems okay since it only affects docs builds and not production code.

Signed-off-by: Stephen Finucane <stephen@that.guru>