[FW][IMP] Livechat: update participate

[fw-bot]

Update Live Chat: participate doc for 19
updating images and content to reflect style rules

Forward-Port-Of: #12983

[robodoo]

[fw-bot]

@jero-odoo @samueljlieber cherrypicking of pull request #12983 failed.

stdout:

Auto-merging content/applications/websites/livechat/participate.rst
CONFLICT (content): Merge conflict in content/applications/websites/livechat/participate.rst
warning: Cannot merge binary files: content/applications/websites/livechat/participate/leave-channel.png (786b97c7ddce4248fd583edd74a3f2f433fe7b81 vs. 1ebf0002fca4c0c89362737e7c82c6251a3bc79a)
Auto-merging content/applications/websites/livechat/participate/leave-channel.png
CONFLICT (content): Merge conflict in content/applications/websites/livechat/participate/leave-channel.png
warning: Cannot merge binary files: content/applications/websites/livechat/participate/managing-chat-responses.png (786b97c7ddce4248fd583edd74a3f2f433fe7b81 vs. 1ebf0002fca4c0c89362737e7c82c6251a3bc79a)
Auto-merging content/applications/websites/livechat/participate/managing-chat-responses.png
CONFLICT (content): Merge conflict in content/applications/websites/livechat/participate/managing-chat-responses.png
warning: Cannot merge binary files: content/applications/websites/livechat/participate/menu-bar.png (786b97c7ddce4248fd583edd74a3f2f433fe7b81 vs. 1ebf0002fca4c0c89362737e7c82c6251a3bc79a)
Auto-merging content/applications/websites/livechat/participate/menu-bar.png
CONFLICT (content): Merge conflict in content/applications/websites/livechat/participate/menu-bar.png
warning: Cannot merge binary files: content/applications/websites/livechat/participate/online-chat-name.png (786b97c7ddce4248fd583edd74a3f2f433fe7b81 vs. 1ebf0002fca4c0c89362737e7c82c6251a3bc79a)
Auto-merging content/applications/websites/livechat/participate/online-chat-name.png
CONFLICT (content): Merge conflict in content/applications/websites/livechat/participate/online-chat-name.png

Either perform the forward-port manually (and push to this branch, proceeding as usual) or close this PR (maybe?).

In the former case, you may want to edit this PR message as well.

⚠️ after resolving this conflict, you will need to merge it via @robodoo.

More info at https://github.com/odoo/odoo/wiki/Mergebot#forward-port