Keep 4.5.1 documentation available somewhere

Is your feature request related to a problem? Please describe.

I expect I am not the only user of pydoc-markdown who is using 4.5.1 before the major changes that came with 4.6.0 (which seem like such a big change that perhaps it should be 5.x.x?). The problem is the documentation for 4.5.1 is no longer available on readthedocs.

Describe the solution you'd like

Make the documentation for 4.5.1 available again (eg. on readthedocs with a notice that documentation for 4.6.0 and later is on github.io or as versioned documentation on readthedocs or on a GitHub branch).

Describe alternatives you've considered

I can get to the old README using the GitHub tag and the md files used to generate the documentation.

Additional context

Another possibility to consider is to keep 4.5.1 in a separate repo since 4.6.0+ is really a complete rewrite with what appear to be different aims rather than an incremental development.

NiklasRosenstein / pydoc-markdown

Keep 4.5.1 documentation available somewhere #266