Bug 1847046 [wpt PR 41320] - docs Dockerfile: Use python:3.11 base image, a=testonly

Bug 1847046 [wpt PR 41320] - docs Dockerfile: Use python:3.11 base image, a=testonly

Automatic update from web-platform-tests
docs Dockerfile: Use python:3.11 base image

Instead of using a somewhat old Ubuntu release as base and installing its
somewhat old Python package, use a base image that already has Python at the
same version we also use for the GitHub workflow that generates
documentation.

"python:3.11" is equivalent to the latest 3.11 release installed on Debian
bookworm (aka stable at the moment of writing).

I have tested this locally and the documentation generated locally finally
looked similar to the published version -- there seemed to be some issue
with Ubuntu 20.04's Python 3.8 and docutils and/or Sphinx that added an
extra ":" after e.g. "Arguments" in the documentation.

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wpt-commits: 89b0e123c1d078944ee38a6e79382dbfc1accfea
wpt-pr: 41320