Continued docs

docs/develop.md

docs/index.md

docs/install.md

docs/publish.md

docs/static/img/badge-appstore.png

docs/static/img/badge-fdroid.png

docs/static/img/badge-googleplay.png

docs/static/img/overview.gif

docs/static/img/overview.mp4

docs/static/img/screenshot-web-notifcation.png

docs/static/img/screenshot-web.png

docs/subscribe/api.md

docs/subscribe/phone.md

docs/subscribe/web.md

examples/subscribe-go/main.go

examples/subscribe-php/subscribe.php

mkdocs.yml