docs: add API, index, and QuickAdd usage documentation

docs/docs/QuickAdd.md

docs/docs/api.md

docs/docs/index.md

docs/docs/resources/demo.gif

docs/docs/resources/episode_list.png

docs/docs/resources/player.png

docs/docs/resources/podcast_grid.png

docs/docs/resources/podcast_note.png

docs/docs/resources/podcast_search.png

docs/docs/resources/quickadd_create_note_settings.png

docs/docs/resources/quickadd_example_note.png

docs/docs/resources/quickadd_timestamp_podcast.png