gnutls improvements

Page: https://ubuntu.com/server/docs/explanation/crypto/gnutls/

- commands and their output should be separated into two codeblocks and '$ ' not part of the copied command content.
- link to cipher defaults "in the case of Ubuntu Jammy" should be updated.
- `j-server.pem` `j-server.key` source unclear
- the "practical examples" to limit ciphers are a bit redundant & complex, could likely be improved.

Start with the GnuTLS page at the linked URL and review its command examples, cipher-defaults link, and references to j-server.pem and j-server.key. Separate commands from output, remove '$ ' from copied commands, update the Jammy link, clarify the certificate sources, and simplify redundant cipher-limiting examples; the page should address each listed concern.

Written by the indexing model from the issue text.