Documentation: update and future improvements

[Laurence Urhegyi <Laurence Urhegyi codethink co uk>]

Hi,

After some MRs have landed this week, we now have pretty solid documentation for 
newcomers to the project. See here:

https://buildstream.gitlab.io/buildstream/

I think we now need to focus on improving the documentation for the more 
advanced features of the the tool, and so I have opened the following ticket to 
capture this:

https://gitlab.com/BuildStream/buildstream/issues/437

I've suggested a way to structure this, which is essentially having tutorials to 
show the different use cases that BuildStream caters for, which get more complex 
as the reader goes through them:

* Building a project
* Developing a project
* Working with Buildstream in a team
* Advanced Projects

Any feedback on this is welcome, as I think it's important to keep going with 
the docs and land these improvements as soon as we can, ideally before GUADEC, 
of course.

Thanks,
Laurence