Create initial documentation for the CladeTime class in virus-clade-utils

[bsweger]

Background

We've been using the CladeTime class in several variant-nowcast-hub scripts and would like to make this class more accessible to other people.

Thus, before adding more CladeTime functionality, let's document what is already there.

Definition of done

• [x] The public-facing functions and classes in the CladeTime class are documented with examples
• [x] Documentation is auto-generated/updated via Python docstrings
• [x] Documentation link is added to pyproject.toml

(2024-10-11: removed the requirement for hosting on GitHub pages...that is too much for one issue.