Use some regexes to reduce the number of lines in .htaccess and
hopefully make it slightly clearer which groupings of files redirect to
which new locations.

Also collapse one redundant double-redirect.

Change-Id: I65c4960856985d71076291f175df17f27a5ab8cc

Once [1] fixed plugin doc generation, the (single) page it produced was
unusably huge. This commit splits it into one page per project.

Note that there are four plugin projects that didn't have sections
included:

- cue
- murano
- tripleo
- watcher

These were noted in hidden rst which is preserved in the (new) index
page.

[1] Id9be39971110fd2eb4519a0582c9bf2514cdcacd

Change-Id: I0214ddb00a5a292a46d7cfb539d6dcc540fdae79
Story: #1735016
Task: #37239

Change-Id: I413a6d059f4eb751ca5c8b5a9b61740ac93680b1