Update to docs root pages and navigation

I think it requires a silly number of steps to get to our
documentation today. It's not huge, but it's "click documentation,
click "current manuals", click 13". And the actual /docs/ page is
mostly empty, which makes it a waste.

PFA a patch that changes this:

* The /docs/ page now gets direct links to all supported documentation
versions, including PDFs

* The /docs/ page gets a big button that sends you directly to
/docs/current/ -- that link wasn't  available at all before

* Translated manuals were listed both on the left and right sidebar on
the root docs page, but not on the manuals page. Remove them from the
left sidebar, leave them on the right one since it's now on the root
docs page.

Unrelated to the restructuring, but while at it:
* Make the left column in the manuals table not take 50% of the width
when all it has is a number

* Add a link to the developer docs

* Remove the repetitive "comprehensive manual" text (it's no more
comprehensive than the other manual, it's just a different format),
and insert a proper separator between the two PDF versions

* Remove some redundant and incorrect template tags probably resulting
from previous copy/paste mistakes

I'm also attaching a screenshot of the main docs page with the
changes, for those that can't easily test a website patch. (Nevermind
the actual versions listed, as this is off a dev version of the
database and not up to date)

Thoughts?

-- 
 Magnus Hagander
 Me: https://www.hagander.net/
 Work: https://www.redpill-linpro.com/