Add version support to published docs

[Vye]

Hi,

Can versioning be added to the docs? I'm a new user and went through a fair amount of unnecessary steps to figure out custom page extension listed in docs is for an unreleased commit in main. :(

Extension changed: https://github.com/dbrgn/tealdeer/issues/320

No .md extension docs for 1.6.1 tag: https://github.com/dbrgn/tealdeer/blob/v1.6.1/docs/src/usage_custom_pages.md

With .md extension live docs with new extension: https://dbrgn.github.io/tealdeer/usage_custom_pages.html

Thanks for the client!