Update documentation to make it easier to start

[lfoppiano]

This PR aims to make the documentation less scary for python and young developers 🐉

Goals:

1. Provide an easy "getting started"
2. Allow people to try on the cloud in advance
3. For local deployment prioritise Docker over the native installation

[coveralls]

coverage: 40.576%. remained the same
when pulling 36ee763 on improvement/better-simpler-documentation
into f6b522b on master.

[lfoppiano]

One comment is to streamline the docker image naming, which is pretty confusing.

We could use

1. grobid/grobid:{version}-{full|crf}
2. or we could use lfoppiano/grobid:{version}-{full|crf} which would be easier for me to maintain :-)