Missing documentation and types for BrowserClient

Nobody has claimed this yet.

The usage docs here:

https://github.com/mapbox/mapbox-sdk-js#usage

Don't mention anything about needing to pass in the "BrowserClient" when using services from a web app, e.g. :

```
import mbxClient from "@mapbox/mapbox-sdk/lib/browser/browser-client";
import mbxGeocoding from "@mapbox/mapbox-sdk/services/geocoding";

const baseClient = mbxClient({
accessToken: process.env.REACT_APP_MAPBOX_ACCESS_TOKEN,
});
const geocodingService = mbxGeocoding(baseClient);
```

And then, once you figure that out, typescript complains about the browser-client import because there are no typings for that module :(

Start with the usage documentation at the linked repository URL and the browser-client module imported as @mapbox/mapbox-sdk/lib/browser/browser-client. Document the BrowserClient setup required for browser services and add TypeScript typings for that import. Done means the browser usage is explained and the import no longer produces a missing-typings error.

Written by the indexing model from the issue text.