Improve the ergonomics of adding custom emoji to the message input

[petyosi]

Motivation Supporting custom emoji, while possible, is quite complex. It involves configuring the picker correctly, injecting the right token in the input, and processing the message so that the custom emoji are correctly displayed.

Proposed solution I've created a custom emoji example as an immediate solution based on customer requests. While I don't have a clear idea on the perfect solution, I think we can integrate the most complex solutions as built-in parts of the codebase + better documentation. For example:

• the custom emoji can be exposed as a prop
• the logic of inserting something when custom emoji is clicked can be exposed as a prop
• the substitution of emoji markers (i.e. :emoji:) is quite complex
• this solution above does not integrate with the auto-complete-textarea : popup.

Acceptance Criteria A proper mix of docs and props so that our integrators do not need assistance to integrate custom emoji.

[arnautov-anton]

Blocked by #1715.