Thanks to DM-15472, we have a lsst-task-topic that marks a topic as being a task topic that is the canonical documentation for a task.

We now want to create toctrees of these task topics that automatically find task topics by some criteria, and then present those topics in a useful way.

Some features:

• Be able to both generate toctree entries and a non-indexing list. This is so that we can create polyhierachies.
• Present the tasks using a definition list structure so that the "term" is the task class's name and the definition is the one-sentence summary or customized summary presented as the content to the lsst-task-topic directive.
• Filter for tasks based on a root Python namespace
• Filter for tasks based on tags (tags are entered on the lsst-task-topic directive).