docs: clarify extension terminology ambiguity (#3881)

docs: clarify 'extension' terminology ambiguity

Add a terminology note section to explain the difference between:
- MarkedExtension: the configuration object passed to marked.use()
- TokenizerAndRendererExtension: custom syntax definitions in the extensions array

This addresses the confusion described in #3182 where 'extensions' is used
with two different meanings throughout the documentation.

Fixes #3182

Co-authored-by: njg7194 <njg7194@github.com>