Make the navigation sidebar versioned

[jin]

Fixes: #6937

This adds versioning support to the navigation sidebar in the default documentation page layout, conditionally loading a sidebar partial based on the current version's major version, or use the latest one in the case for rolling releases. This also means that the navigation sidebar list is extracted out from the documentation.html layout file, into an include partial.

This gets rid of the many conditionals littered all over the navigation list items.

Currently, the biggest differences are between the groups of v0, v1-3, and master. This implementation allows v1 and v2's sidebar to just be a symlink to v3. For rolling releases, the sidebar is versioned as master.

% tree _includes/documentation-sidebar
_includes/documentation-sidebar
├── 0
│   └── sidebar.html
├── 1 -> 3
├── 2 -> 3
├── 3
│   └── sidebar.html
└── master
    └── sidebar.html

We can also allow finer-grained versioning with the full major.minor.patch, but I think major versions are a good enough start.

[meisterT]

Thanks for working on this Jin! What's the expected workflow on updating this? Should we add an item to https://github.com/bazelbuild/continuous-integration/blob/master/docs/release-playbook.md?

[jin]

Depending on whether we want sidebars versioned on the major version or the full version, the release manager would just need to create a new symlink or new directory in the versioned sidebar doc.

Alternatively we have a generate_versioned_docs setup script, can probably add something in there..

[floriographygoth]

LGTM.