Part of New IA implementation #267:

• /docs/next/:
  • Install page from reworked dl-build page - Installation page (docs/next) from reworked download page #294, Install page (/next), final adjustments #323
  • Rename file and add redirect rule - Rename docs/next/dl-build to install #319
• /docs/v3.4/, i.e., /docs/latest: Install page updates for versions older than 'next' #333
  • Install page from reworked dl-build page
  • Rename file and add redirect rule
  • Add Install link to top-nav
• All other versions: Install page updates for versions older than 'next' #333
  • Rename all dl-build pages to install, so that the versions menu will work more smoothly
  • Change page titles to be Install
  • Add redirect rules