Use absolute cross-references in docstrings - #323

Since mkdocstrings-python 2.0.9, relative cross-references are expanded
from the path where an object is defined, not where it is documented.
All our public symbols are defined in private modules and re-exported
from `frequenz.dispatch`, so 6 of the current references can't be found
anymore and the strict documentation build fails, blocking the bump.

Counting the dots from the private module would fix the build, but it
ties the references to the private module layout, breaks them on older
mkdocstrings-python versions, and can make references to inherited
members silently link to the wrong class. Relative references are also
hard to read in an IDE, and only work in projects rendering our
inherited docstrings if they enable `relative_crossrefs` too.

All relative references are converted to absolute references to the
public path of their target, which don't depend on any of that. The
targets were taken from what mkdocstrings-python 2.0.8 expanded each
reference to, and the built documentation has the same links as
before, with both mkdocstrings-python 2.0.8 and 2.0.10.

The references in the docstring of the private
`DispatchScheduler.new_running_state_event_receiver()`, which is not
rendered, are converted too, pointing to the public symbols they were
meant for.

Some paragraphs are rewrapped to fit the longer references.

See frequenz-floss/frequenz-repo-config-python#655 for details.

Signed-off-by: Leandro Lucarella <luca-frequenz@llucax.com>