Re #7777 #7778: check requirements.txt for security advisory

There is a new make file,

    doc/Makefile

that defines these goals:

  - `check-requirements`:
    Check `requirements.txt` for security problems (CVEs) using `skjold`.
    This goal is intended for the "Users guide" CI.
    SKJOLD_GITHUB_API_TOKEN might have to be set if GITHUB_TOKEN is
    not in the environment, in order to access the GitHub GraphQL API.

  - `build-and-check-requirements`:
    Rebuild `requirements.txt` from `requirements.in` using `pip-compile`,
    and check with `check-requirements`.
    This goal is intended for manual invocation.

    It is invoked from the top Makefile via goal
    `users-guide-requirements`.

Alternatively, these goals could be coupled with the doc build
`make users-guide`.
However, since these goals require a couple of seconds to run, I think
it is annoying to call them on every build of the documentation.