API and developer documentation

I suggest to add the following markdown documents and reference them from the top-level `README.md`:

```
Docs/user_guide/modules/pipelinecreator.md
Docs/user_guide/modules/pipelinecaseiterator.md
Docs/developer_guide/modules/pipelinemodules.md
Docs/developer_guide/modules/pipelineclibridge.md
```

For `pipelinecreator.md` and `pipelinecaseiterator.md`, consider looking at examples in https://github.com/Slicer/Slicer/blob/master/Docs/user_guide/modules/, for example [endoscopy.md](https://github.com/Slicer/Slicer/blob/master/Docs/user_guide/modules/endoscopy.md)

Indeed, I expect the UI will have minor changes.

For `pipelinemodules.md` and `pipelineclibridge.md`, you can look at examples at https://github.com/Slicer/Slicer/tree/master/Docs/developer_guide/modules. For these ones, I will keep things brief with a high level sentence and few links to relevant classes.

Example of content for `pipelinemodules.md`
```
# PipelineModules

This module is intended to provide the API for implementing module wrappers and registering them with the `PipelineCreator` module.

## Type of wrappers

[...]
```

### Why this documentation structures ?

This is motivated by https://github.com/Slicer/Slicer/issues/6090

_Originally posted by @jcfr in https://github.com/KitwareMedical/SlicerPipelines/pull/14#discussion_r803157421_

Review the requested files under Docs/user_guide/modules and Docs/developer_guide/modules, then compare their intended structure with the linked Slicer examples. Check the top-level README.md for the appropriate references and inspect the relevant module APIs and classes. Done means all four documents exist with the requested user or developer guidance and are linked from README.md.

Written by the indexing model from the issue text.