Document HttpClient usage

The BDK proposes a generic [`HttpClient`](https://github.com/finos/symphony-bdk-java/blob/main/symphony-bdk-http/symphony-bdk-http-api/src/main/java/com/symphony/bdk/http/api/HttpClient.java) that allows developers to perform calls to external systems.

The advantage of this `HttpClient` is it relies on the same implementations that are used to perform calls to the Symphony REST APIs.

Also, developers are free to choose between different implementations by simply selecting the right dependency:
```xml

org.finos.symphony.bdk
symphony-bdk-http-jersey2
runtime

org.finos.symphony.bdk
symphony-bdk-http-webclient
runtime

```

## What do we want?

- [ ] create new `http-client.md` in [./docs/](https://github.com/finos/symphony-bdk-java/tree/main/docs) folder
- [ ] reference this file in [`index.md`](https://github.com/finos/symphony-bdk-java/blob/main/docs/index.md)
- [ ] document configuration and usage of the [`HttpClient`](https://javadoc.io/doc/org.finos.symphony.bdk/symphony-bdk-http-api/latest/index.html) interface and its different implementations