doc: process, sending_patches: Update and correct the old "Patches" content

- Use gender-neutral language to refer to the user, consistently.
- Reference the checkpatch document.
- Move the section on commit message tags to the process document and
  reference this in sending_patches.rst.
  - Reword the custodian workflow process section to refer to this new
    section, integrate some of the wording from there in this new section.
- Update the comment about GPLv2 applying to August 2022, to be clear
  this still is correct.
- Reword the section about MAKEALL to talk about local build testing and
  link to the CI document.
- Reference the system_configuration document for the note about
  modifying existing code.
- Reword the patchwork flow section.

Cc: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Tom Rini <trini@konsulko.com>