Add landing page for debian installer download

[Jesse Rhodes <jesse@sney.ca>]

Hi Installer team,

I have a merge request pending for webwml and I was suggested by Paul
Wise to see if you had any feedback.

Currently, the "Download" button on debian.org points directly to the
netinst iso. This MR would replace it with a page that still initiates
an automatic download of netinst, but also points to verification
checksums, the install guide, and other useful stuff.

The merge request along with a screenshot can be seen here:
https://salsa.debian.org/webmaster-team/webwml/-/merge_requests/699

And I have a (semi) working demo of the new download page at:
https://debian.sney.ca/download.en.htm

Please let me know what you think! I am also reachable on IRC (sney,
on both OFTC and Libera) if you'd like to chat directly.

Thanks for your consideration,

Jesse (sney)