Split "Install from source" documentation section

[jnsebgosselin]

The idea would be to split the current section named Install PyHELP from source in two and have a section named Install PyHELP from source and another section named Contribute to PyHELP.

In the Install PyHELP from source section we would not talk about git at all. We will also link to an official release of PyHELP to download the source code.

In the Contribute to PyHELP we will give instruction to run PyHELP from the development version using git. We also need to add instruction to be able to build the documentation. Also, we need to add instruction to install the required dependencies for the tests.