docs(how-to): Add upstream patch submission document from old wiki

[RossPorter506]

Add a document explaining how to go about submitting a patch to the upstream Linux kernel mailing-list.
Also add an explainer and some differentiation between the Linux kernel mailing-list and the Ubuntu mailing-list pages.

• Added or updated meta description (if this PR includes documentation changes)
• Reviewed all GitHub Actions / CI results; any unresolved failures are explained in a comment

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[RossPorter506]

Link check is a false positive (it's a weird Matrix link so I'm not surprised), spell check is also a false positive (unless I've forgotten how to spell).
Removed URLs test seems broken.

[bjdean-canonical]

Thanks @RossPorter506

Link check is a false positive (it's a weird Matrix link so I'm not surprised)

You can add r"https://matrix.to/" to docs/conf.py linkcheck_anchors_ignore_for_url to fix this check.

spell check is also a false positive (unless I've forgotten how to spell).

You can add "maintainer's" to .custom_wordlist.txt where words are ok, or use the spellexception" in-line role if it's not a common word

Removed URLs test seems broken.

This test succeeded on re-running.