SELFDEV-576 + DEVDOCS-5863: [organize] Make docs structure match IA, move newer public docs to public repo

[slsriehl]

SELFDEV-576 + DEVDOCS-5863

What changed?

• Make docs structure match IA
• BOPIS and promotions docs into public repo
• Other than adding the spec files from bopis and promotions to the dev center, i did not reorganize the API reference because that requires comms and advance notice to everyone who maintains an API client or SDK.

Release notes draft

• The bigcommerce/docs repo now contains the OAS for promotions, inventory, locations, and other newer API features!
• It's now easier than ever to contribute to the docs. The source file structure in the repo now matches the Dev Center URLs. Finding the doc you want to modify never had less friction :)
• This only applies to the narrative docs. API reference source file restructure is in the works as soon as we can give enough notice for all you API client maintainers to reset your references.

Anything else?

ping @bc-tgomez @bc-traciporter

[slsriehl]

I made some of the retext changes, particularly typos. Changing more is beyond the scope of this work, and retext will fail given the number of files in the PR, for rate limiting reasons.