Tutorial-style "introduction" to the Overview section

[Jose Moreira (joseemoreira)]

This is a new "introductory" subsection to the "Overview" section, to make the document more readable.

[Philipp Tomsich (ptomsich)]

Jose Moreira (@joseemoreira) This is failing whitespace checks:

trim trailing whitespace.................................................Failed
- hook id: trailing-whitespace
- exit code: 1
- files were modified by this hook