Expose a ini file for edit online in Collabora Office / LibreOffice

The idea is to have a button inside of Collabora Online / LibreOffice that allows the user to switch to editing online by clicking it.

Requirement is that richdocuments would implement https://github.com/nextcloud/richdocuments/issues/2418
- Provide an endpoint with a path parameter to open the file for the user
- Expose the endpoint URL through capabilities, so that it can be picked up by the client

The desktop client would expose a `.editonline.ini` (naming could change) file in the root sync folder which contains the edit url as well as the userid and relative path, which can then be read by Collabora Office.

- [ ] Fetch the endpoint from the capabilities
- [ ] For each sync folder put a .ini file in the root, containing the format below

### Example 1

The Nextcloud root is synced to ~/Nextcloud, there would be a ~/Nextcloud/.editonline.ini:

```
[editserver]
endpoint = https://nextcloud.example.com/index.php/apps/richdocuments/editfromlocal
userid = admin
relativepath = /
```

Considering a file Document.odt is opened, Collabora Office could then construct url to open in the browser:
https://nextcloud.example.com/index.php/apps/richdocuments/editfromlocal?user=admin&path=/Document.odt

### Example 2

The Nextcloud subdirectory /Engineering is synced to ~/Nextcloud, there would be a ~/Nextcloud/.editonline.ini:

```
[editserver]
endpoint = https://nextcloud.example.com/index.php/apps/richdocuments/editfromlocal
userid = admin
relativepath = /Engineering
```

Considering a file Document.odt is opened, Collabora Office could then construct url to open in the browser:
https://nextcloud.example.com/index.php/apps/richdocuments/editfromlocal?user=admin&path=/Engineering/Document.odt

Start by tracing how the desktop client fetches capabilities and represents each sync folder; no specific files or tests are named in the issue. Confirm the related richdocuments endpoint contract, then verify that each sync-folder root gets a correctly populated .editonline.ini and that paths for root and subdirectory files produce the expected browser URL.

Written by the indexing model from the issue text.