Created new guide doc for the registerCompany mutation

DEVDOCS-6473

What changed?

• The BigCommerce's GraphQL Storefront API now has a mutation registering new Company accounts as an existing storefront customer.
• This PR adds a guide for using the registerCompany mutation, and adds a reference to the new guide in the REST Storefront Companies API reference article.

Release notes draft

• Created a new article to cover the registerCompany mutation for B2B Edition storefronts.

Anything else?

ping {names}