Update layout of documentation navigation pages

Update layout of documentation navigation pages

* Move links to current manuals to /docs/, decommission /docs/manuals/
* Add link to /docs/current/ with a large button
* Add links to /docs/devel/ at the bottom of the version table
* Remove translated manuals from left nav menu, since they are in the
  right one
* Remove redundant wording around "comprehensive manual"
* Clean up some incorrect template tags

Discussion: https://www.postgresql.org/message-id/CABUevEymaWDL4JywHxBDBATUUPPrAf4_tGwin4CtGFdQ5tx5Hg@mail.gmail.com