Versioned code links

[ajm188]

Right now, there's no (good?) way to have links to vitess code correspond to the doc version being documented.

For example, I would like to have a doc for a piece of code that in v16 docs points to github.com/vitessio/vitess/blob/v16.0.0/some/path and in v17 docs points to github.com/vitessio/vitess/blob/v17.0.0/some/path

Of course I can do this manually, but this breaks when we do the full copy to start prepping for a new version