Transparent "whiteboard" : Draw on shared screen or shared document

### Your use case

#### What would you like to do?
Be able to interact (draw, write...) on the shared document when implemented (https://github.com/vector-im/element-call/issues/633) and on the shared screen.
#### Why would you like to do it?
Someone is sharing something (a document or his screen) to explain a situation (an issue).
Others want to be able to show some ideas to help or explain something to the person who is sharing the document/screen and for that need to draw a line, draw some arrow, add some colours on the screen like a sketch, add some text.
At the end of the meeting we can take a "screenshot" of all interactions, all contributions.
#### How would you like to achieve it?
I guess we need to add a layer, like a shared transparent whiteboard over the shared screen/document and allow users (maybe some only, the moderator should decide about that) to draw on it.

### Have you considered any alternatives?

Bigbluebutton is offering something like that. But it would be nice to have it within Element call within Element app family.

You could get inspiration also from https://wbo.ophir.dev/?lang=en but with a transparent background to be able to see the shared document/screen. Whiteboard is nice also. It could be a solution to put a screeshot of the shared document/screen and put it easily on the background of the whiteboard.

### Additional context

_No response_