Documentation: explain Activity Embeds for users & developers
Split @vapvarun’s PR to create docs/developer/components/activity/embeds and docs/user/components/activity/embeds.
Update the Activity Embed Block’s documentation link in favor of the updated one.
Deprecate docs/developer/group-extension/README.md in favor of docs/developer/components/groups/extension leaving a link to the second documentation page in the first one.