docs: archivist special remote has no meaningful documentation (online)

[mslw]

The docs page for the archivist special remote only has one sentence, and does not contain any details.

It seems that those pages are generated from the module docstrings (top of the file). Comparing to the uncurl special remote:

• annexremotes/archivist has a one-sentence module docstring and a longer ArchivistRemote class docstring;
• annexremotes/uncurl has an extensive module docstring and an empty UncurlRemote class docstring.

At the moment I am not sure if we should simply move UncurlRemote docstring out of the class definition (feels like it - but to leave the class without a dosctring?), or play with Sphinx's autosummary / autodoc settings.