Add health check endpoint (#18465)

Add health check endpoint (#18465)

* chore: add health check endpoint

docs: update document about health check

fix: fix up Sqlite3 ping. current ping will success even if the db file is missing

fix: do not expose privacy information in output field

* refactor: remove HealthChecker struct

* Added `/api/healthz` to install routes.

This was needed for using /api/healthz endpoint in Docker healthchecks,
otherwise, Docker would never become healthy if using healthz endpoint
and users would not be able to complete the installation of Gitea.

* Update modules/cache/cache.go

* fine tune

* Remove unnecessary test code. Now there are 2 routes for installation (and maybe more in future)

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: 6543 <6543@obermui.de>
Co-authored-by: Marcos de Oliveira <marcossantos@furb.br>