Removing old/deprecated API specs and updating README file

[SujitGadkari]

• Removing API references for Invoices V1, Payments V1 and Partner Referrals V1 specs
• Updating README file to remove above spec references along with Orders V1 which was removed by Yash

This is based on Docs content team's recommendation.

[yashranipa]

clean!

[johnpyp]

I noticed this from a month ago while looking for the Payments V1 schema.

Payments V1 is "deprecated", however it's still in use with non-deprecated apis.

Specifically, the PAYMENT.SALE.COMPLETED event is the official webhook event for Subscription notification handling:

The schema of that webhook resource is a sale from Payments V1.

As such, I needed to get to the schema to be able to have proper type handling in my openapi codegen'd entities.

I think the schemas for these deprecated APIs should probably remain they're still in use in the flow of handling non-deprecated APIs.

I'd be happy to move over to the Payments V2 entities, except that the webhook still hasn't moved! 😅