Moves the "Hacking on Gitea" page out of the documentation website and into the repository as `docs/development.md`, so contributors find build and test instructions next to the code. The content has been cleaned up and corrected for in-repo use.
---------
Signed-off-by: bircni <bircni@icloud.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
bircni