Enhance documentation versioning

[wshayes]

Would be better to version the entire language website for each version of BEL in case any of the supporting website content changes with a new version. The current versioning only supports the BEL Specification content.

Background

• https://github.com/jaegertracing/documentation has an approach for this - they build all of the versions into one documentation site with one version identified as latest.
• https://zayd62.github.io/mkdocs-versioning/0.3.0/