Unify Installation and Getting started guides

[luisfabib]

In the documentation, for a new user, it is not clear how to first navigate the page to obtain the information needed to get a first example running.

Right now under Installation there are two categories: Source-Code version and Executable. However, in the landing page there is no mention of the differences between the two and even points out to an external webpage (for the download, see #1).

The Getting started section seems to be repeated for both the Source-Code version and Executable sections. Also, it would be helpful to provide the expected response of the program to the example MMMx distributions.mcx recommended by the guide. This examples seems to be recommended as a form of sanity check that the installation was correct, so it would be good to provide what output should the users expect.