Docs Site: display docs versions from previous releases

[danbudris]

What would you like to be added: Support multi-version doc site

Why is this needed:

Currently, our doc site only supports single version (see screenshot below).

However, I would like to request multi-version support for a couple reasons:

• from supporting customer perspective: while we roll out newer versions, our doc site will also get updated to drop off out-of-date contents, while they can still be beneficial to many users;
• from UI perspective: when I see a version drop-down button like above, I expect it to be a list of versions that I can select from.