Flexible indexes refactoring notes (#4979)

* add flexible indexes refactoring notes

* include some of the PR comments in the notes

* update / integrate items from the PR discussion

* fill index creation, access, propagation sections

* fill index use / encode / repr sections

* fill remaining sections + tweaks

* Update FLEXIBLE_INDEXES_NOTES.md

Co-authored-by: Ryan Abernathey <ryan.abernathey@gmail.com>

* Update FLEXIBLE_INDEXES_NOTES.md

Co-authored-by: Ryan Abernathey <ryan.abernathey@gmail.com>

* Update FLEXIBLE_INDEXES_NOTES.md

Co-authored-by: Ryan Abernathey <ryan.abernathey@gmail.com>

* Update FLEXIBLE_INDEXES_NOTES.md

Co-authored-by: Ryan Abernathey <ryan.abernathey@gmail.com>

* address review comments

* address more review comments

* [skip-ci] move notes to design_notes subdirectory

Co-authored-by: Ryan Abernathey <ryan.abernathey@gmail.com>