Selective Sync usability improvements

follow up for #911 listed in https://github.com/lutraconsulting/input/pull/1572

A) mergin-config.json location
Since Input ignores hidden files ATM, mergin-config.json is located in the project directory. The best would be to hide it either to .mergin folder or just make it as a hidden file, but with ability to sync. Note that it can be modified over time although Input suppose to only read it.

B) The initial download use case
When an Input client downloads a project for the first time, the mergin-config is suppose to be downloaded as well, thus selective sync is not configured at that state. The best would be request mergin config from a server in advance and then continue with download/sync having mergin-config context already set up. Or, request a mergin config as the very first file for the download if its listed in remotely added files. Moved to #1590

C) New status for images in the photo widget~~
When some photos are excluded from the sync and therefore not present on client's drive, but actually are used as value for an attachment field, the external resource widget shows it as missing image. The better would be to notify user that the image is stored on server and it only hasn't been synced on the user's device due to selective sync.

D) add documentation to the user docs

E) add user-tests

The remaining work concerns the photo widget's external-resource handling and selective-sync state; start by locating those entry points in the mobile app. Verify how an excluded image is currently reported, then make the completed behavior distinguish a server-stored image that was not synced from a genuinely missing image.