Add Makefile

[hugovk]

Based on https://github.com/python/devguide/blob/main/Makefile with a lot of unnecessary stuff removed.

This makes it easier to update the documentation:

❯ make help
Please use `make <target>' where <target> is one of
  venv       to create a venv with necessary tools
  html       to make standalone HTML files
  htmlview   to open the index page built by the html target in your browser
  htmllive   to rebuild and reload HTML files in your browser
  clean      to remove the venv and build files
  linkcheck  to check all external links for integrity
  lint       to lint all the files

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📚 Documentation preview 📚: https://docs-community--123.org.readthedocs.build/

[egeakman]

Nice! Thank you ❤️

[ezio-melotti]

There is already a Makefile in docs/.

[hugovk]

Ah, so there is! Shall we delete that one?