Improve and rewrite the documentation

[thoughtpolice]

What needs to be done

The documentation needs a rework and update; in particular several of the features implemented since the initial draft are completely undocumented (like JSON .narinfo results or re-signing mode.)

Furthermore, I've come to determine that while I like org mode as an Emacs user, I dislike it as an actual format for project documentation (or at least GitHub frontmatter), so the README and CHANGELOG need to be rewritten as well.

I think it would be nice to use something like Sphinx and GitHub Actions to synchronize things appropriately so e.g. https://thoughtpolice.github.io/eris worked.

Alternatives considered

None: this must be implemented.

Additional notes

None.