Move legacy js reference

[nik9000]

Right now we generate the reference material for the legacy branches of the javascript client as "version 16.x" which is sort of weird because the modern version is 7.x. We'd like to do two things:

1. Move the legacy branch's url from 16.x to legacy.
2. Create a redirect from 16.x to legacy.

This is difficult right now because we don't own the redirects. But we're working on that! We should fix this issue once we have full control of the redirects.