Docs Request: Improve documentation around Querying against an @index (GSI) for Android API Category

### Before opening, please confirm:

- [X] I have [searched for duplicate or closed issues](https://github.com/aws-amplify/amplify-android/issues?q=is%3Aissue+) and [discussions](https://github.com/aws-amplify/amplify-android/discussions).

### Language and Async Model

Not applicable

### Amplify Categories

GraphQL API

### Gradle script dependencies

N/A

### Environment information

N/A

### Please include any relevant guides or documentation you're referencing

https://docs.amplify.aws/lib/graphqlapi/query-data/q/platform/android/ and https://docs.amplify.aws/lib/graphqlapi/advanced-workflows/q/platform/android/#nested-data

### Describe the feature request

Add some docs describing how to query for non-standard queries against defined indexes.

Basic usecase ends up returning null for field values, but you can serialize to a string to retrieve data, getting the paginated results is a little more complicated, and not documented today.

Retrieving as a string: https://gist.github.com/alharris-at/4ab46c9f2bb96aae2045064753e187eb
Deserializing to a PaginatedResult: https://gist.github.com/alharris-at/e34341bdb7fd197d5d8ceecee5bc2070

### Initialization steps (if applicable)

_No response_

### Code Snippet

See Gists

### amplifyconfiguration.json

_No response_

### GraphQL Schema

```graphql
// Put your schema below this line

```

### Additional information and screenshots

_No response_

Start with the Android GraphQL API querying guide and its nested-data section, then review the two linked gists for the documented usage. The documentation is done when it explains querying defined indexes, retrieving values through string serialization, and deserializing paginated results into PaginatedResult.

Written by the indexing model from the issue text.