Added API description for the Payments: IAP & Subscriptions plugin

[Ombuweb]

Added the API section describing all the methods offered by the plugin.

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[rigor789]

Hi @Ombuweb thanks for the PR, could you open the PR against this file: https://github.com/NativeScript/payments/blob/main/packages/payments/README.md as the contents in this repo are updated daily from the plugin readmes (so your changes here would disappear in ~24hours).

Sorry for the confusion - we plan to make this better when we get the chance...

[Ombuweb]

Hi @rigor789, no problem. I will open it against the other file.

[Ombuweb]

One question @rigor789, does that mean I should use the packages' README.md when contributing to packages docs and docs-new for other contributions?

[rigor789]

Yep, plugins are separate and just pulled in daily on CI

The rest is managed here