Documentation

[paoloambrosio]

Write user documentation including an introductory guide, hooks (random execution order), contexts, and any other feature.

Edit: reworded and got rid of what was done as part of other tickets

[konserw]

Should we also add something there:

1. https://github.com/cucumber/docs.cucumber.io ?
2. https://github.com/cucumber/api.cucumber.io ?

[paoloambrosio]

Reopen if we ever decide to do it.