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.

[RossPorter506]

Thanks, I added those changes and squashed them into the original commit.

[bjdean-canonical]

Perfect thanks

[bjdean-canonical]

@RossPorter506 apologies - I merged a conflict fix earlier on (in send-patches.rst) which has now caused another conflict. Could you please rebase and resolve that (should just be choosing the new title and note below:

-How to send patches to the mailing list
-#######################################
+How to send patches to the Ubuntu mailing-list
+##############################################
+
+The Ubuntu mailing-list is the place to submit Ubuntu-specific changes, such as Ubuntu kernel configs. 
+
+.. note::
+    If your patch otherwise applies more generally to the Linux kernel, you should :doc:`submit your patches upstream <./send-patches-upstream>` instead.

[RossPorter506]

Let me know if there's anything else, otherwise it would be good to get this merged before there's more conflicts with main.