Documentation should have a slug that resolves to the latest tag of the project.

[shymega-ct]

(I'm opening this issue in BuildStream, buildstream-plugins, and buildstream-plugins-community, as it'll need implementing across those)

It would be nice to have a special slug in the documentation website tree, where the special type of 'latest' would show the docs for the latest version of the project.

This would be useful for documentation that always needs to refer to the latest released version of the project.