Backport of Use venv in CI (#916) to humble

[mergify]

Description

Fix this error:

 error: externally-managed-environment

× This environment is externally managed
╰─> To install Python packages system-wide, try apt install
    python3-xyz, where xyz is the package you are trying to
    install.
    
    If you wish to install a non-Debian-packaged Python package,
    create a virtual environment using python3 -m venv path/to/venv.
    Then use path/to/venv/bin/python and path/to/venv/bin/pip. Make
    sure you have python3-full installed.
    
    If you wish to install a non-Debian packaged Python application,
    it may be easiest to use pipx install xyz, which will manage a
    virtual environment for you. Make sure you have pipx installed.
    
    See /usr/share/doc/python3.12/README.venv for more information.

This should make CI pass the "Install Python Dependencies" step.

---

This is an automatic backport of pull request #916 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 1709ece has failed:

On branch mergify/bp/humble/pr-916
Your branch is up to date with 'origin/humble'.

You are currently cherry-picking commit 1709ece.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   doc/how_to_contribute/how_to_contribute_to_site.rst
	modified:   htmlproofer.sh

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   .github/workflows/deploy.yml

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally