Consolidate docstring and documentation

[ericpre]

The problem is also, that it is not consistent how detailed the file_reader and file_writer docstrings are. For some formats more is documented in the user guide, for others more in the docstrings.

Originally posted by @jlaehne in #35 (comment)

[ericpre]

One approach could be to:

• put the generic/introductory plugin specific documentation a rst/md file for each plugin, which could live in the docs folder or in the plugin folder
• add the file_reader and file_write docstrings in the plugin specific page of the docs

This could be done by one of the two:

• use a sphinx template
• include the API reference in the plugin specific documentation page.

Considering that sphinx is a bit brittle and not always clearly documented, using the sphinx template approach may be more difficult but it would have the advantage of enforcing consistence between plugins and avoid repeating code.

[jlaehne]

* put the generic/introductory plugin specific documentation a `rst`/`md` file for each plugin, which could live in the `docs` folder or in the plugin folder

That file exists for every plugin afaik, in docs/supported_formats/.

* add the `file_reader` and `file_write` docstrings in the plugin specific page of the docs

That remains to be done, though some plugins have very detailed docstrings and others none at all.