Improve Discoverability of our "Embed Mode" #1093

concerning: https://docs.opencloud.eu/docs/dev/web/embed-mode

Status quo

• The page is a low-level API reference.
• Readers who aren't already implementing it can't tell what it's for.

3 Problems to fix

1. No use-case framing. Add a short "What you can build" section at the top with concrete scenarios, e.g.:

   • "Attach a file from OpenCloud" in a webmail or chat client
   • "Save to OpenCloud" from a third-party app (CRM, form builder, document editor)
   • Demo via collabora -> insert image and text-editor: "save as"

2. Discoverability
   The nav entry "Embed Mode" is not click-friendly or self-explanatory. You need to click on it and read the content to understand what it is about. That is not the purpose of a navigation entry. It should be self-explanatory. I'd call it "File Picker". (We called it "Embed Mode" to better distinguish it from the old file picker.)

3. Visuals

   • Add at least one screenshot or video of the embedded UI in each mode.