Revamp the docs

[reyammer]

• update main README, for now with a simple callout to the new *-rc python and rust packages
• update python/README.md with info about the new module (and with a callout about the old docs (pointing to old commits of the repo) and changelog)
• remove docs/python.md and update all the links to python/README.md
• update python/CHANGELOG.md
• add a "Contributing" section in the main README
• expand the rust's client docs with quick example, help, and a couple of more advanced examples (with json output and input from a url, eg curl https://google.com | magika - -s)
• Update the citation snippet
• Consider merging ./docs/magika_output.md into ./python/README.md
• Consider merging ./docs/command_line.md into ./rust/cli/README.md
• Have a self-contained migration doc section for the python API differences