Improve portability of docs automation

[jakob-keller]

          FWIW, it's nice to run the same automation @ RTD, CI, and locally. I like using tox for this (some people go for nox).

You can make RTD call tox as follows:

$ cat .readthedocs.yaml
# Read the Docs configuration file
# See https://docs.readthedocs.io/en/stable/config-file/v2.html for details
---

version: 2

build:
  os: ubuntu-24.04
  tools:
    python: >-  # has to be parsed as a YAML string
      3.11
  commands:
  - >-
    PYTHONWARNINGS=error
    python3 -Im venv "${READTHEDOCS_VIRTUALENV_PATH}"
  - >-
    PYTHONWARNINGS=error
    "${READTHEDOCS_VIRTUALENV_PATH}"/bin/python -Im
    pip install tox
  - >-
    PYTHONWARNINGS=error
    "${READTHEDOCS_VIRTUALENV_PATH}"/bin/python -Im
    tox -e build-docs --notest -vvvvv
  - >-
    PYTHONWARNINGS=error
    "${READTHEDOCS_VIRTUALENV_PATH}"/bin/python -Im
    tox -e build-docs --skip-pkg-install -q
    --
    "${READTHEDOCS_OUTPUT}"/html -b dirhtml
...

Another example: pytest-dev/pytest-asyncio#920

This would allow wiring such a self-install into tox that could be run locally and in any other envs the same way.

Originally posted by @webknjaz in #1165 (comment)

[jakob-keller]

          Remember to add `-n -W --keep-going` to the invocation in a follow-up.

Originally posted by @webknjaz in #1189 (comment)