change git repo link from anonscm.debian.org to salsa.debian.org

See merge request !8

also: be more verbose on how to create a separate text file,
how to sign it and how to include it in an email

The public keyserver network is mostly not useful these days.
nm.debian.org points folk directly at the Ubuntu keyserver, so do the
same.

Gunnar has switched from 0x673A03E4C1DB921F to 0x2404C9546E145360 and
the old key is now expired, so update the example keyring commands to
use the new key rather than one which won't work.

Federico Grau authored May 22, 2021 and Jonathan McDowell committed Jun 14, 2021

Removed stale warnings.  Updated gpg output to current version (from 1.x
to 2.2).  Made clear these commands were last reviewed circa Debian 10
buster.

Update the anchor to point to the correct section

See merge request !3

The section #s3.7 does not exists anymore, its now called #retiring.
This also changes the name of the html document in the link from
'ch03.en.html' to 'developer-duties.en.html', because thats what the
page redirects to.

Link to salsa.d.o, as anonscm.d.o is deprecated

See merge request !2

add ids to the different sections of replacing_keys

See merge request !1

so that then I can link
    https://keyring.debian.org/replacing_keys.html#untrusted-key



Signed-off-by: Mattia Rizzolo <mattia@debian.org>

 * fixed unescaped < and > chars.
 * balanced tags

Use https where possible, and fix weird and broken URLs.