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: !229 (comment 40999)

Ideas:

• keep cargo xtask gen-doc but remove generated files from git index
• 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
• modify the xtask doc template to link to the doc generated with cargo doc