DOP-4784-B: Remove chatbot FAB from 404, use ChatbotUi bar

[mmeigs]

Stories/Links:

DOP-4784 (b)

Current Behavior:

Landing
Cloud-docs
404

Staging Links:

Landing
Cloud-docs
404

Notes:

This is a re-do of DOP-4784.

I had previously added the Chatbot FAB to the 404 page. What is actually needed is the ChatbotUI bar.

This PR removes the FAB from the 404. It also adds the ChatbotUi component to the 404 page.

In doing so, there were styling differences between the landing template and the errorpage template that needed to be addressed to match the design requirements listed in the ticket's comments. The chatbot bar needed to match the width of the page elements which currently are simply justified to center no matter the width of the div. The changes in this PR cement the widths of the divs in the main section so that the chatbot bar can match their widths in all screensizes.

README updates

• • This PR introduces changes that should be reflected in the README, and I have made those updates.
• • This PR does not introduce changes that should be reflected in the README

[rayangler]

Thanks for working on this! Changes generally look good. Just wanted to ask some questions before formally approving

[rayangler]

We might want to check in with Sarah Lin to make sure these changes are okay, but otherwise LGTM