Annotate and document anchor.py (#817)

* Annotate and document `anchor.py`.

* Format fixes by ruff

* Add documentation for parents.

---------

Co-authored-by: knutnergaard <15194233+knutnergaard@users.noreply.github.com>
Co-authored-by: Ben Kiel <ben@typefounding.com>

Author: knutnergaard