Documentation updates

[benjaoming]

• Some of the most important usage instructions were buried under .. include:: ../README.rst, which was a shame.
• There were some mistakes in the documentation about related objects - they are related to subscriptions, not notifications.
• ...I speed-wrote a new how-to in #132 about relations between notifications and other model objects: https://django-nyt.readthedocs.io/en/latest/howto/object_relations.html

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (fe68568) 74.87% compared to head (eb7c28a) 74.87%. Report is 1 commits behind head on main.

:exclamation: Current head eb7c28a differs from pull request most recent head 7c2693c. Consider uploading reports for the commit 7c2693c to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #133 +/- ## ======================================= Coverage 74.87% 74.87% ======================================= Files 23 23 Lines 621 621 ======================================= Hits 465 465 Misses 156 156 ```

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