Issue 429 [No Ticket]: add POST bulk pricing rules

[bc-andreadao]

[No Ticket]

PR created to address https://github.com/bigcommerce/docs/issues/429

Relevant slack conversation

Screenshot proof:

What changed?

• Add create bulk pricing rules method to documentation
• Improve structure of yml file for bulk pricing rules

Release notes draft

• Update documentation to include Create a Bulk Pricing Rule endpoint

Anything else?

@bigcommerce/team-catalog

[bc-andreadao]

Thanks @olysenko