docs(ChatPane): Add attachments and link to chat pane messages in prototype

[sophieH29]

This PR aims to improve chat pane behavior by adding Attachments and links to chat message so provide the proper accessibility experience for it.

1. Navigate with UP / DOWN arrow keys between messages
2. Click Enter on focused message
3. The link will get focus first
4. Press Tab
5. First attachment will get focus
6. Press Tab
7. Action button on the attachment will get focus
8. Press Enter
9. Popup will open with context menu and trap the focus
10. Press Esc
11. Focus goes to action trigger
12. Press Tab and go to the next attachment OR press Esc and focus message again, so you'll be able to navigate between messages with UP/DOWN arrow keys

Opened context menu:

[codecov]

Codecov Report

Merging #514 into master will not change coverage.
The diff coverage is n/a.

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##           master     #514   +/-   ##
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  Coverage   88.17%   88.17%           
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  Files          42       42           
  Lines        1455     1455           
  Branches      212      212           
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  Hits         1283     1283           
  Misses        167      167           
  Partials        5        5

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[layershifter]

Left one small note, LGTM

[DustyTheBot]

	Warnings
⚠️	There are no updates provided to CHANGELOG. Ensure there are no publicly visible changes introduced by this PR.

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[mnajdova]

The changes looks good, have tested it locally and it is behaving as in the description. However, it is a bit overwhelming for all messages to have attachments. Can you at least add the attachments on every 5th message let's say? Will approve after this change. No other changes required from my side.