docs: Config Migration Guide (#17230)

* docs: config  migration guide

* first full draft of page

* Fix lint errs

* Add set up section

* add reviewer feedback to migration guide

* copy edits

* Apply suggestions from code review

Co-authored-by: 唯然 <weiran.zsd@outlook.com>

* implement feedback

* Apply suggestions from NZ code review

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

* implement reviewer feedback

* copy edit

* Apply suggestions from code review

Co-authored-by: Francesco Trotta <github@fasttime.org>
Co-authored-by: Nicholas C. Zakas <nicholas@humanwhocodes.com>

* remove example subheadings

* Apply suggestions from code review

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

* linter options section

* Apply suggestions from code review

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

* Update flat config import explanation

* Apply suggestions from code review

Co-authored-by: Francesco Trotta <github@fasttime.org>

* Fix indentation

* Apply suggestions from code review

* Apply suggestions from code review

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

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Co-authored-by: 唯然 <weiran.zsd@outlook.com>
Co-authored-by: Nicholas C. Zakas <nicholas@humanwhocodes.com>
Co-authored-by: Francesco Trotta <github@fasttime.org>
Co-authored-by: Milos Djermanovic <milos.djermanovic@gmail.com>