Allow read/write to user root and only read to group git on documenta…

[bitmand]

The Installation from binary documentation has this section:

If you don’t want the web installer to be able to write the config file at all, it is also possible to make the config file read-only for the gitea user (owner/group root:root, mode 0660), and set INSTALL_LOCK = true...

But setting app.ini to root:root and 0660 will only allow root user to read it and gitea can't read app.ini and fails to start.

This MR changes the documentation to recommending:

(owner/group root:git, mode 0640), and set INSTALL_LOCK = true

Allow root user to read/write and users in git group read-only access.

[6543]

🚀