[Data Loader] [Docs] Add chain loader implementation and related docs

[robfrawley]

Q	A
Branch?	2.0
Bug fix?	no
New feature?	yes
BC breaks?	no
Deprecations?	no
Tests pass?	yes
Fixed tickets	#1011
License	MIT
Doc PR

This PR re-adds the chain loader implementation originally merged into 1.0 (as #953) but never merged into 2.0 (see #1011 (comment) and the continued conversation for more context).

Original PR Description

Chain data loader implementation. This allows one to define one "filter set" that searches multiple loader configurations (for example, this allows you to check both a Gaufrette stream loader and a file system loader simultaneously).

Here is an example configuration for the loader:

liip_imagine:

    loaders:

        foo:
            filesystem:
                # setup this loader here

        bar:
            flysystem:
                # setup this loader here

        baz:
            chain:
                # name the loaders to chain
                loaders: [foo, bar]

    filter_sets:

        my_custom_filter_set:
            data_loader: baz
            filters:
                # define your filters here

This implementation simply loops through the available loaders and selects the first valid result, so if the file exists in multiple loaders, the first defined in your configuration will be selected. For example, if file.ext exists in both the foo and bar loader, the one found via the foo loader will be returned because it is defined first for baz.chain.loaders.