[DOCS] Differentiate API and SDK documentation

[brad-dow]

The current Open Payments documentation does not clearly distinguish between the API and SDK content. The technical content focuses on SDK implementations without adequately introducing the SDK.

• Review and analyze existing documentation to identify sections that discuss the API versus those that reference the SDK
• Create clear distinctions between API and SDK documentation, making sure that each section effectively communicates its purpose and usage
• Update the documentation structure where applicable (changing SDK snippets to SDKs, etc.) to enhance clarity and usability for developers

[brad-dow]

PR created to address this.

The PR did not include changes to any of the "Guides" pages. I know these are still being worked on so I didn't want to introduce anything that would conflict with work in progress. I could see either some introductory information being added to each of the guide pages, or perhaps an overview-style page like we have for the SDKs section. This page could mention that these guides are walking through usage for one or more of the SDKs, and maybe even some of the shared steps across guides could be updated in this overview-style page--things like dpendencies, HTTP message signature partial, etc.