docs: add documentation for autodoc

[paullatzelsperger]

What this PR changes/adds

Adds documentation for the autodoc plugin.

Why it does that

Documentation is good :)

Further notes

List other areas of code that have changed but are not necessarily linked to the main feature. This could be method signature changes, package declarations, bugs that were encountered and were fixed inline, etc.

Linked Issue(s)

Closes # <-- insert Issue number if one exists

Checklist

• added appropriate tests?
• performed checkstyle check locally?
• added/updated copyright headers?
• documented public classes/methods?
• added/updated relevant documentation?
• assigned appropriate label? (exclude from changelog with label no-changelog)
• formatted title correctly? (take a look at the CONTRIBUTING and styleguide for details)