Improve documentation for external developers

[tbsbdr]

Context

Turned out that the current dev docs are not yet fully sufficient for external developers to integrate the filepicker without help from us (ex. from @kulmann ).

Expected outcome

• An external developer with little knowledge about ocis can integrate the filepicker in his/her application within reasonable time and without having to ask for help owncloud internal developers.

Learnings so far

• It was not clear to external devs what the filepicker can and can not do
  • It was not understood that the upload is not handeled via the filepicker
  • It was not understood that the filepicker returns solely a resource descriptor
• A practical example guide about how to call ocis apis subsequently to complete a location or filepicker scenario would have been helpful

[rzatoc]

Hopefully it is ok to add a case to this issue:

A fellow developer discovered the following:

It turns out, there was a breaking change in the file picker API. There is no dist/wc/file-picker.min.js file anymore. The file mentioned in the documentation[1] does not work at all, first because unpkg tries and fails to load version 0.1.8 of the file picker, and second because the file dist/wc/file-picker.js is an ES module (and doesn't load even with type="module").

[...]

Also, there is no dist/wc/img/ directory anymore. Which files besides the main file-picker.* are required for version 0.11?

[1] https://github.com/owncloud/file-picker/blob/master/docs/installation.md?plain=1#LL74C1-L87C4