website: resolve "latest" to latest version in shortcodes (#3436)

This came up in the discussion of #3426.

Note that there's two ways to access the current "latest" version:

https://www.openpolicyagent.org/docs/latest/http-api-authorization/
https://www.openpolicyagent.org/docs/v0.28.0/http-api-authorization/

For both sites, the shortcodes now resolve to "0.28.0". Before, the
first one would show "latest", the second "0.28.0".

Signed-off-by: Stephan Renatus <stephan.renatus@gmail.com>