doc: develop: process: Document using b4 and patchwork for custodians

- We already have good custodian documentation for patchwork, add a
  reference and then link to it here.
- Add a reference to the existing b4 documentation, and reference it
  here.
- Note and link to patchwork integration, am/shazam and ty features of
  b4 as these are the most likely useful portions. Be specific about
  keeping the default ${summary} as that includes important information.

Signed-off-by: Tom Rini <[email protected]>
Reviewed-by: Mattijs Korpershoek <[email protected]>
Reviewed-by: Quentin Schulz <[email protected]>

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