Draft comparison between MarlinPC and SonicPC

[weikengchen]

Description

This is a draft version of the comparison between MarlinPC and SonicPC, with some additional information extracted from the individual modules, so it can server as a README for the src directory.

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Before we can merge this PR, please make sure that all the following items have been
checked off. If any of the checklist items are not applicable, please leave them but
write a little note why.

• Targeted PR against correct branch (master)
• Re-reviewed Files changed in the Github PR explorer

N/A:

• Linked to Github issue with discussion and accepted design OR have an explanation in the PR that describes this work.
• Wrote unit tests
• Added a relevant changelog entry to the Pending section in CHANGELOG.md
• Updated relevant documentation in the code