Fix versioning of library

# Library versions too granular

Library docs shouldn’t be re-released for minor patch releases, like 1.0.0 and 1.0.1. There should only be versioning for major releases.

# No versioning for Ignition releases

Currently, the library docs are versioned for new releases of the library itself, but do not indicate which version of Ignition each version belongs to.

The “general” docs (/docs) are also not versioned, resulting in multiples of the same pages (installation, ros integration, comparison, etc.) residing in the same repository.

The complete documentation should be versioned under Ignition release names. The address taxonomy could look something like this:

Where only the “/”s corresponding to a certain release would be available to view when that release is selected.

Whether implementing this is practically possible (presumably using scripts to pull docs from different repos, and different branches within those repos, depending on the branch built in the original repo) would need to be determined.