Documentation updates

[lemon24]

User guide:

• Replace list(reader.get_entries())[:2] with reader.get_entries(limit=2) throughout.
• (maybe) Add guidance (on naming etc.) for plugin authors.
• User-added entries (Some feeds have only the last X entries #239).

Read me:

• Mention that reader helps users track their consumption (via count averages, and read/important_modified).

API docs:

• Use literals instead of str for argument types in docstrings.
• Types for Entry, Feed etc. attributes aren't showing.
• Use Sphinx to show exception inheritance.
• As of August 2023, instance attributes are missing for exceptions (e.g. FeedError is missing url).