Docs: rest_api: add documentation for map and filter processing steps

[burnash]

Description

Since dlt-hub/verified-sources#495 is merged it is now possible to use map and filter functions on the resources generated by rest_api:

config: RESTAPIConfig = {
    "client": {
        "base_url": "https://api.example.com",
    },
    "resources": [
        {
            "name": "posts",
            "endpoint": "posts",
            "processing_steps": [
                {"map": lambda x: x["id"] * 10},
                {"filter": lambda x: x["id"] > 10},
            ],
        },
    ],
}

Task

Update rest_api source documentation covering use cases for this feature. Use relevant tests from dlt-hub/verified-sources#495 to derive code examples.