* Add 'watch-backend'

This leverages `air` to watch the backend files and trigger `make
backend` automatically when they change. It seems to work rather well
together with `watch-frontend`.

Fixes: https://github.com/go-gitea/gitea/issues/12318

* rework docs to a new section for continuous build

Co-authored-by: techknowlogick <techknowlogick@gitea.io>