MINIFICPP-2021 Auto-generate extra doc sections in PROCESSORS.md

[fgerlits]

https://issues.apache.org/jira/browse/MINIFICPP-2021

This is a follow-up on #1483, broken up into two parts to make reviewing somewhat more manageable.

After this PR, everything in PROCESSORS.md is auto-generated except for the Predefined Groups section in PerformanceDataMonitor, which is unlike any of the other sections, and I didn't want to invent a new category just for this. Maybe it could be added to the Description, but it seemed too large for that. Let me know what you think.

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