Chat UI quality of life improvements

[JasonWeill]

Fixes #55 (remaining tasks)

Adds a placeholder message for the chat box:

Adds a tooltip for the send message button:

Updates a screen shot in the docs.

[dlqqq]

It looks like this PR also includes many other changes from other branches; could you rebase the pertinent commits back onto main? For example:

git checkout main
git pull
git checkout -
git rebase HEAD~3 chat-ui-tweaks --onto main
git push -f

[JasonWeill]

@dlqqq Revised to exclude changes in #136.

[andrii-i]

This looks and works well.

At the same time if we want Jupyternaut to be seen as another collaborator in RTC chat environment, more neutral chat input placeholder at L48 like "Send a message" would work better. Call to action can then be part of a placeholder (as well as instructions and example prompts) that is shown when there are no messages in the chat (separate PR). I might be missing context here so feel free to ignore.

[dlqqq]

This looks great! Awesome work @JasonWeill 🎉

@andrii-i These are good suggestions, but right now, the AI always responds to the user. So the placeholder "Ask Jupyternaut anything" makes sense. Once we implement multi-user rooms that require the AI to be mentioned before responding, then we can change the placeholder. 👍