docs: update Satusehat overview documentation

[hunterachieng]

Description

Update satusehat adaptor overview documentation

Fixes #509

[josephjclark]

@hunterachieng I came here from the example docs. A couple of thoughts:

1. I find it odd that there's no mention of FHIR at all. I don't know a lot about satusehat but as we're using FHIR with it, and a lot of our examples are FHIR, it seems like this is something worth talking about. Even if only briefly.
2. This overview section would be a good place to post a full FHIR resource example - not the short versions we show in the Functions page, but a full Resource

Also, would this be a good place to:

a) Link to the postman collection, as a useful learning resource
b) Carefully explain how the HTTP helpers map to the requests defined in postman? Or more generally, how the HTTP helpers build a URL?