Update encrypted messengers with instructions for Signal, Element, Jami, Briar & Silence

Content request

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Checklist

• This issue is not a duplicate, I've searched the Issue Tracker
• This issue is not a question, I'm aware that I can submit questions via Mastodon, Twitter, Email, Reddit or Matrix
• This issue is a content request, and not a bug report, feature request, or anything other

What are you missing or what could be improved?

Update section on encrypted messengers (Signal, Element, Jami, Briar & Silence).

What is the solution you'd like to see?

1. provide comparison of privacy respecting messenger apps
2. look at xmpp
3. provide clear, detailed instructions for Signal, Element, Jami, Briar & Silence

Do you have additional comments?

Related to Issue #27 (closed)