Refactor tasks documentation

[sergei-maertens]

Currently task documentation is mostly visible via the /taskmappings endpoint, which poses a number of problems:

• long list of configured tasks, lack of overview/grouping
• only displays tasks that are connected to a topic, not all available tasks

One of the goals is to have BPTL documentation available for non-technical people so they get an idea of which generic building blocks are available. A grouping of the tasks per topic/domain (the python packages are already structured this way!) in the presentation layer would provide a first improvement.

Other improvements are:

• documentation (aspects) in Dutch
• more textual and less technical documentation

A new documentation section should facilitate this. We can add this to the existing setup and gradually replace existing pages.

Acceptance criteria:

• A new entrypoint/page for documentation
• All available work units' documentation can be viewed, even if not connected to a topic
• Work units are grouped per domain (ZGW, Camunda, ValidSign...)
• Documentation is extracted from docstrings - limited to general descriptions.

Nice to have:

Improved documentation parsing/system to document work units. A decorator-like approach could be viable, something like:

@register
@task.variable("zaakUrl", "URL reference to the zaak", required=True, type=str)
def example_task(task: BaseTask):
    do_stuff()
    return {}

but this could also be handled by function ArgSpecs or other formats (Sphinx arg docs for example).