I've just seen someone expect that uploading a source package would be enough.

The existing one, with Tails-specific packages, is now called our "custom APT
repository". On top of that, we'll soon have time-based snapshots and tagged
snapshots of upstream APT repositories.

This commit adjusts a small part of this documentation, but mainly:

 * reworks the files and directories layout;
 * adjust links accordingly;
 * adds XXX placeholders where new documentation must be written, or existing
   documentation must be updated.

This avoids having to use a special merge driver when building on Jenkins, and
in general should make most merges "just work" as intended, minimizing both the
risk of merge conflicts that need to be dealt with manually, and the risk of Git
wrongly resolving merges automatically.

Contributors doc: make it clear that overlay APT suites are *not* added when building a tagged release.

It does *not* contain the full list of APT suites we'll be adding
dynamically at build time, so let's make it clearer that it "merely"
contains a list of overlays (that will be used in addition to e.g.
the base branch's APT suite).

It is meant to change, but let's get ourselves a clean starting point :)

Its UI is way nicer than repo.or.cz, it is easier to configure, it offers HTTPS,
and most importantly we can easily push there ourselves (to get an up-to-date
mirror, and e.g. remove old tags or rewrite history) as opposed to our old
mirror-style repo on repo.or.cz.