docs: Custom Rules cleanup/expansion (#16906)

* Start copy edits

* redo page IA

* copy edits

* copy edits

* Apply suggestions from code review

Co-authored-by: Nicholas C. Zakas <nicholas@humanwhocodes.com>
Co-authored-by: Nitin Kumar <snitin315@gmail.com>

* shebang guidance

* reconcile changeS

* Apply suggestions from code review

Co-authored-by: Nitin Kumar <snitin315@gmail.com>

* Apply suggestions from code review

* fix broken inline links

* highlighting for types

* Apply suggestions from code review

Co-authored-by: Milos Djermanovic <milos.djermanovic@gmail.com>
Co-authored-by: Nitin Kumar <snitin315@gmail.com>

* implement MD feedback

* Apply suggestions from code review

Co-authored-by: Nicholas C. Zakas <nicholas@humanwhocodes.com>

* implement MD feedback

* Update docs/src/extend/custom-rules.md

Co-authored-by: Nitin Kumar <snitin315@gmail.com>

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Co-authored-by: Nicholas C. Zakas <nicholas@humanwhocodes.com>
Co-authored-by: Nitin Kumar <snitin315@gmail.com>
Co-authored-by: Milos Djermanovic <milos.djermanovic@gmail.com>