DM-15821: New directives for listing task topics

[jonathansick]

The lsst-tasks, lsst-cmdlinetasks, lsst-pipelinetasks, lsst-configurables, and lsst-configs directives create listings of topics that are marked with lsst-task-topic or lsst-config-topic roles.

These listings not only link to the topic, but also show a summary that's either extracted from the corresponding docstring or set through the lsst-task-topic or lsst-config-topic directives.

These directives also generate a toctree, optionally.

Example usage in lsst.pipe.tasks:

Task reference
==============

Command-line tasks
------------------

.. lsst-cmdlinetasks::
   :root: lsst.pipe.tasks

Tasks
-----

.. lsst-tasks::
   :root: lsst.pipe.tasks
   :toctree: tasks

Configurations
--------------

.. lsst-configs::
   :root: lsst.pipe.tasks
   :toctree: configs

And here's some of the output: