RFC: Pinning Document(s) when using chat UI

[cwang]

Is your change proposal related to a problem? Please describe.

Right now we're operating based on spaces which contain documents. Another way, which people find useful, is to go from a set of documents first and run questions against them.

The assumption here is that users know which documents to start with. This offers a more targeted use which is helpful for summarisation etc.

Propose the solution you'd like

Step by step:

1. Allow users to select a set of documents from the document list of a space
2. They click "query" or something similar as an in-context drop-down menu
3. It switches back to the Chat UI with an indication that this particular question is within the pre-selected context
4. Get the answer as always

A further enhancement is to allow selections from documents that live in different spaces.

Describe alternatives you've considered
N/A

Additional context
Users may be able to do it today with spaces created only for this particular set of documents.