Improve documentation

[swaptr]

The crate looks in good shape but still suffers from poor documentation. Let's fix it!

• [x] Readme and docs/index.mdx update @swaptr
• [x] Hacking guide @vincenzopalazzo
• [x] Installation guide @vincenzopalazzo
• [x] Generic introduction on website @swaptr
• [ ] License @vincenzopalazzo
• [x] Open a mailing list for discussion @vincenzopalazzo
• [ ] Adds a list of maintainers and reviews @vincenzopalazzo
  • N.B: There are open positions :)

[vincenzopalazzo]

I should work on finishing it