Add versioning to the left hand-side menu

[jmineraud]

Hi,
similarly to the languages (in fact, in addition to the languages), it would be cool to have the versioning of the documentation. Any ideas on how to enable this ?

[eine]

I this is difficult because not all the information is available when you build a single version. Say you have versions A and B. You build both of them to a subdir, both with a template that accepts a list of strings (["A", "B"]) and displays a drop-down menu (where each item points to a subdir). Then, you upload it to say GitHub Pages. Now you need to build a new version, C. You build it with argument ["A", "B", "C"], and upload it to subdir C in the target gh-pages branch. As a result, it will be possible to go from version C to either A or B, but none of these will be aware of C.

A possible workaround is to rebuild all the versions when a new one is added to the list. However, this is not desirable because each version might require a specific version of hugo and/or the theme.

A different solution is to use a JSON file that contains an updated list of available versions and a some Javascript that generates a menu dynamically. When a new version is added, the JSON file needs to be modified only. This approach is used in https://github.com/buildthedocs/sphinx_btd_theme/blob/24f6805417e2c127f6ffa371906f7cbc95d7c4d2/src/theme.js#L213-L232. The advantage is that, when the JSON file is not found (or JS does not work), the version is shown as a simple element (instead of a menu).