v3.5 docs prep: rename next to v3.5

[chalin]

From #328 (comment):

I'd like to propose the following strategy, at least until we decide whether #252 will be addressed or not:

1. Rename /docs/next to /docs/v3.5
2. Add a redirect rule from next to v3.5 (and remove the opposite redirect rule that was added via Prepare for v3.5 docs -- add a redirect link #326)
3. NOT create another /docs/next folder until we have a need for it and/or until we've resolved what we'll be doing with Use only real version names, not next, for doc version subdirectories under /content #252, among other issues.

Originally posted by @chalin in #328 (comment)

[chalin]

It occurred to me that this might cause trouble/break the versions menu, but hopefully not. We might need to address #322 earlier.