It enables to not depend on swh-web to build standalone swh package
documentation, plus these sphinx features could be reused in other
package documentation.

Related to T3258

- add the scheduler journal client
- remove the indexer journal client (for clarity)
- remove indexer workers (for clarity)
- add a postgresql db attached to the web app

Closes T3229.

It seems to be misunderstood by most people when they start out.

It is now required for swh-web frontend development.

And add environment variable SPHINXOPTCOLOR to disable it on Jenkins

The command is not always installed by default, for instance in the
debian:buster docker image.

These django settings contains the deposit ones but some swh-web settings
are also needed to avoid errors when running autodoc.

Closes T3248

Installation is implicit when using tox but not in a custom make script.

The new pip resolver ends up backtracking numerous packages when
installing all depencies in a single pip command but this is
terribly slow and long to execute.

A workaround is to install each swh package one at a time by
executing pip multiple times, no backtracking happens in that
case and it reduces considerably the overall installation time.

Another advantage of proceeding like this is that it ensures each
swh package will be installed in development mode.

Also increase tox verbosity when calling make to have some feeback
of what it is doing.

It used to only document the core components.

This commit does the following:

* expands the existing descriptions
* reorders them in didactical order
* and all the other components in two new tiers ("major" and "extra", in
  addition to "core")

This extra requirement has been dropped from swh-storage.

I think it's a little clearer this way what Keycloak is
and when a reader should care (or not) about it.

To match the toctree.

I'm leaving keycloak out of it, because it does not actually describe
the architecture. It's only in the architecture toctree for lack of
a better place for now.

This only includes redirects for user documentation;
external links to the developer documentation would be broken,
but I don't think there are any such link and I don't want to
clutter the file tree unnecessarily.

So the docker documentation is available on docs.softwareheritage.org instead
of linking to the source on Phabricator

This adds the roadmap entry in the site in the:
- index menu on the left (prior to the components listing)
- main page under a ` Roadmap` section just before the components section

Related to T3208

Dependencies must be written in topological order, or they will be fetched
from PyPI instead of the local system

This will make linking from module documentation to this page easier.

This also fixes the deposit dependency.

Related to T3172

Stargirl Flowers' guide mostly codifies what we are already do, and has
some great advice on using mocks that we should follow.

It add a lot of noise to the graph, because it depends on literally everything.