docs: allow linking to current released version docs pages without a version number

[ianmcook]

For example it should be possible to link to a URL such as:

• https://arrow.apache.org/adbc/cpp/index.html
• https://arrow.apache.org/adbc/latest/cpp/index.html
• https://arrow.apache.org/adbc/stable/cpp/index.html

(whichever convention we prefer) and that should currently redirect to https://arrow.apache.org/adbc/0.5.1/cpp/index.html.

[lidavidm]

current might be best given latest is ambiguous and we aren't stable. And I'd rather not pollute the root directory so an explicit namespace would be best.

We could do this easily enough by just copying the docs into both directories. The tricky part would be finagling how it looks in the version switcher. We'd also want to set up the redirect to go to current and not main.