Document supported ingest paths for Obs data

[theletterf]

As hinted by @lucabelluccini, we have at least 11 different ways to ingest data into Obs.

We should document which ones we support and recommend, and make sure they're documented.

Create an "equivalence table of ingest solutions" and provide clear guidance on choosing the right option.

[theletterf]

Closing this one in favor of https://github.com/elastic/docs-content-internal/issues/375

+CC @karenzone