Add information about `.writeable` to the documentation about update_manager

[AndyEveritt]

Is your feature request related to a problem? Please describe

Currently adding an update_manager xxx to the moonraker config sets the corresponding path to read only in the web interface using fm.add_reserved_path

This is useful most of the time but it would be useful to be able to override it so the directory is writeable for example when using the update managerto sync config files to a git repo.

I know it is possible to make the directory writeable by adding a file called .writeable in the top level folder, but as far as I can tell, this is not documented anywhere other than the source code.

I am sure there are others that will want to do the same thing but will not have the commitment to check the source code.

Describe the solution you'd like

Add documentation about the .writeable file and what it allows.

Describe alternatives you've considered

No response

Additional information

No response

[Arksine]

This feature is undocumented for good reason, it is primarily for developer use. If you are savvy enough to search the source this then you are likely to understand what modifying a git repo entails. It isn't expected that most users will know how git works.