docs: Custom Formatters page cleanup/expansion (#16886)

* docs: custom formatters cleanup/expansion

* custom formatters page edits

* Apply suggestions from code review

* Apply suggestions from code review

Co-authored-by: Nicholas C. Zakas <[email protected]>
Co-authored-by: Milos Djermanovic <[email protected]>

* implement NZ/MD feedback

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

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

Co-authored-by: Nitin Kumar <[email protected]>

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Co-authored-by: Nicholas C. Zakas <[email protected]>
Co-authored-by: Milos Djermanovic <[email protected]>
Co-authored-by: Nitin Kumar <[email protected]>