Modernise Gerrit intro page

Update the Gerrit Code Review introduction page by removing obsolete
references to Atlassian Jira and give more space to new integrations
such as access to the AI review agent chat.

Also update the social channels links and include X, Mastodon BlueSky
and Discord.

Old intro page: https://www.pasteboard.co/-zxX9NtbQ2Tx.png
New intro page: https://www.pasteboard.co/JD_FF2dDq52A.png

Release-Notes: Update the Gerrit introduction page.
Change-Id: I1abe2669fcb95e22d944a246100b6498e6042c65