Revert "Remove old bundled sidebar template as this is no longer used."

This reverts commit 97da7e10422bd9c8930a6822f1d94ac6de12e912.
Register BP Docs' template directory and use it on the wiki homepage.

If we ever want to use a BP Docs template on the wiki homepage as a
fallback, we are now able to use it.
Filter 'the_title' on the wiki homepage.
Remove 'buddypress' div for template part.
Trim whitespace.