[docs] document plugin/subplugin type deprecation

[snake]

Added:

1. A section to the 5.0 devupdate covering this new feature
2. A section in the Plugin types docs page, detailing deprecation process for plugin+subplugin types
3. Links on the Metadata page pointing to 2.
4. A link in "See also" on the Deprecation page pointing to 2.

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[snake]

OK, bear with me (first docs PR), will address linting issues.