docs: Don't refer to the tracklist as a playlist

(cherry picked from commit 4a6e7d292c7d5fdbf7b30b8f4363d023f072ac87)
docs: Only list web clients on the web extension page
docs: Move Mopify to the pip-installable list
docs: Add Mopidy-AudioAddict extension
docs: Add Mopidy-EvtDev extension
docs: Add Mopidy-Local-SQLite extension
docs: Add Mopidy-OE1 extension
docs: Add Mopidy-Simple-Webclient extension
docs: Add Mopidy-TtsGpio extension
docs: Add Mopidy-WebSettings extension
docs: Add Mopidy-Webhooks extension
Merge branch 'master' into release/v0.19.x