Doc improvements

[balat]

### Tasks
- [ ] https://github.com/ocsigen/ocsigen.github.io/issues/51
- [ ] https://github.com/ocsigen/html_of_wiki/issues/140
- [ ] https://github.com/ocsigen/ocsigen.github.io/issues/53
- [ ] https://github.com/ocsigen/ocsigen.github.io/issues/54
- [ ] https://github.com/ocsigen/wikidoc/issues/18