Updating the homepage

[Arash Esbati]

Hi Mosè,

I think our move to ELPA-only releases is quite complete, one open issue
is the homepage.  Do you see a chance to give it a final touch?  I think
this will be for a long time ;-)

Changes I see are:

• https://www.gnu.org/software/auctex/index.html
  Current AUCTeX version: 13.3 (2024-01-17) should link to
  https://elpa.gnu.org/packages/auctex.html

• https://www.gnu.org/software/auctex/download.html
  should only promote ELPA, and
  https://www.gnu.org/software/auctex/download-for-unix.html
  https://www.gnu.org/software/auctex/download-for-windows.html
  https://www.gnu.org/software/auctex/download-for-macosx.html
  https://www.gnu.org/software/auctex/download-for-other.html
  should be removed.

• https://www.gnu.org/software/auctex/documentation.html
  should link to manuals provided here:
  https://elpa.gnu.org/packages/doc/auctex.html
  https://elpa.gnu.org/packages/doc/preview-latex.html

Anything else I'm missing?  TIA.

Best, Arash