Various clarifications and reformulations based on my experience using
the README to build and run Gargantext. In particular:
- Information related to development has been given its own section
- Docker Compose has been added to the list of prerequisites
- Information pertaining to using Stack has been moved to an annex file
- The content to be copypasted into a `cabal.project.local` file has been moved
  out of the README into a separate file `cabal.project.local_toCopy`

Those may not be the best decisions, I am happy to discuss them if
needed

The service was causing docker compose to fail. It seems to simply no
longer be available.

This commit moves `GargConfig` and the other config-related
data structures back into gargantext, so that they can be edited
and expanded without needing to worry about the prelude project.

Using goldenVsStringDiff creates problems with CI trying to locale
`gargantext-cli -- golden-file-diff`.