RTD no longer automatically generates config files.

The `.readthedocs.yaml` is now required: https://blog.readthedocs.com/migrate-configuration-v2/

The `conf.py` for Sphinx is also required: https://blog.readthedocs.com/doctool-without-configuration-file/
- The previously automatically generated `conf.py` contained
  `sphinx_rtd_theme` -> in order to keep it we have to set it and add it
  as a requirement.
- The version field in `conf.py.in` remains uncofigured but I don't
  think it is a problem because it doesn't shows up in the html docs.

To allow us and others to prepare for switching dnf5 as the default in Rawhide.

Include information about the automated logic for downloading metadata.

See also rpm-software-management/dnf5#1252.

Resolves: https://issues.redhat.com/browse/RHEL-25005

The original code took the first item from all remaining packages.
It means a random reason and use it to keep installonly package reason.

Related: https://issues.redhat.com/browse/RHEL-15902
Closes: #2061

Installonly packages are handled by autoremove command in a similar
way like other package.

Resolves: https://issues.redhat.com/browse/RHEL-15902

Leaf packages term is not define in documentation and it even
represent different set of packages that are showed by leaves command.

Related: https://issues.redhat.com/browse/RHELDOCS-17711

The described behavior differs from current behavior therefore
it might create a confusion.

Related: https://bugzilla.redhat.com/show_bug.cgi?id=922667
Related: https://issues.redhat.com/browse/RHEL-1342

 Matches rpm-software-management/dnf5#1309 for
 DNF 5.

Signed-off-by: Stephen Gallagher <[email protected]>

Use `dnf-3` and `dnf4` files for bash completion and link the `dnf` only when `dnf5` is not used yet.