Document email client

- Document briefly Claws Mail.
- Delete https://tails.boum.org/doc/encryption_and_privacy/openpgp_with_claws_mail/index.en.html
- Make space for #7685 (closed).

Feature Branch: doc/7694-email_client

Related issues

• Related to #8301 (closed)
• Related to #8419 (closed)
• Related to #9159 (closed)
• Blocks #7685 (closed)

Original created by @sajolida on 7694 (Redmine)