GitLab

* Add Preferences library API and tutorial documents

Add API and tutorial documents for the  Preferences library.

* Revise per review

Correct some errors. Remove the "wordiness" is a few places.

* Correct link to Preferences API

Update the references and link to the Preferences API document.
Co-authored-by: Pedro Minatel <pedro.minatel@espressif.com>
Co-authored-by: Me No Dev <me-no-dev@users.noreply.github.com>