Remove internal API reference

[mollykarcher]

The maintained versions of these docs live in https://github.com/stellar/stellar-docs

I did a mindnumbingly laborious passthrough to sanity-check that all of the content here was present in some form in the public developer docs. It appears that there is only one real difference (that I could find), and it's minimal. The public API docs for the Horizon-specific error responses (example) are lacking this attribute-level descriptions in a table format. This format is present in these internal docs and also for the non-horizon specific error responses in the public docs, as well as all successful API responses in the public docs. We can update the public docs with this if we want. But regardless, this seems like a pretty minor difference and there's no major content missing, so I vote we just kill all this off.

I also think we should delete SDK_API_GUIDE. This reads almost like it's the middle section of an onboarding checklist, so I'm guessing some of our old onboarding docs used to reference it. But we have this essence captured in our Platform onboarding docs internally.