Introduce `pdoc` and improve the documentation

Moodle Tools is currently documented in two stand-alone Markdown files. To improve the documentation (especially for the API), we want to integrate the API documentation tool pdoc into Moodle Tools. For that, we need proper doc strings for each moodle, class, and function. The existing Markdown files from docs can then be integrated into the module-level documentation (i.e., the respective init.py files).

• Create type annotations for all classes and functions
• Add pdoc to the development requirements
• Create a GitLab CI automation that creates a new GitLab page with pdoc on every commit