Refac generated documentation
In 1.6.0 substrate upgrade, the call documentation disappeared from runtime metadata. We then have to rethink the way we generate documentation.
See discussion: https://git.duniter.org/nodes/rust/duniter-v2s/-/merge_requests/229#note_40999
Ideas:
- [x] keep `cargo xtask gen-doc` but remove generated files from git index
- [x] generate documentation with `cargo doc` and host it somewhere online, built by a CI
- [ ] add an other CI which runs the gen doc xtask and output the results somewhere for Duniter website to take from
- [x] modify the xtask doc template to link to the doc generated with `cargo doc`
issue