Build and publish docs per version (#158)

* Remove unused files
* Add publishing step
* Allow manual trigger, and fetch full depth
* Refactor actual docs build code into fluent.docs
* Add versions to the project docs.
* Freeze dependencies of the docs build process
* Update Sphinx to 3.5.1

Use classes for builders, use subclassing instead of code-flow
ifs to distinguish the top-level builder from the project builders.