DEVDOCS-6497 - Buyer Portal guides

[bc-terra]

Initial branch creation with relevant guide files.
Base information added to each file.
Guides for Headless and Stencil filled in. Missing screenshots. Other files are placeholders for future content.

DEVDOCS-6497

What changed?

• Old buyer portal guides are deprecated or obscured in technical structure.
• New guides set up to go by steps. Content separated by expected structure.

Release notes draft

• We've created new guides for setting up Buyer Portal covering native Stencil, Headless, and Catalyst deployment with the default build when possible and custom Buyer Portal in any case.

Anything else?

ping { @CNanninga @bigcommerce/dev-docs-team }