Add GPG to email section

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?

Provide instructions for GPG as alternative to encrypted email providers such as Protonmail & Tutanota.

What is the solution you'd like to see?

Dedicated section which explains:

1. How to create a key pair
2. How to back up the key pair
3. How to import public keys
4. How to encrypt emails

Do you have additional comments?

N/A