Clarify docs on the goal you are trying to achieve

As a newcommer, the whole documentation seems to answer the "how?" question, but not the "what?" question. I find that hard to understand because of that, as I have the impression of [cargo culting](https://en.wikipedia.org/wiki/Cargo_cult) without really understanding what I'm doing.

Let's take for example the "Getting Started" page. It tells you "do this, do that", but not the initial problem you're trying to solve. I had the very same feeling when looking at the [FOSDEM 2018 presentation about Conan](https://fosdem.org/2018/schedule/event/conan/). It separates "creating packages" from "consuming packages", but when you create packages you may need to consume others so that's confusing. The difference between conanfile.txt and conanfile.py is thus unclear. Should I have one? Which one? Should I have both? If I have both is there one that is used in priority?

I want to use libraries but also want to package mine, so in which case i'm in?

Start with the “Getting Started” page and the documentation sections on creating and consuming packages. Review the FOSDEM 2018 Conan presentation and the explanations of conanfile.txt and conanfile.py. Done should make the user’s initial goal, package workflows, and choice or interaction between these files understandable.

Written by the indexing model from the issue text.