doc: improve Sign/Verify examples and docs

- Use complete examples that can be pasted and run as-is
- Move note about algorithm to the functions it applies to
- Uncapitalize inconsistence use of "Class"
- Use both EC and RSA keys in the examples
- Note that hash and digest are two names for the same algorithms

PR-URL: https://github.com/nodejs/node/pull/25452


Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: James M Snell <jasnell@gmail.com>