Add Project Overview to documentation

Add Project Overview to documentation

Highly based on article about MuxPi on wiki.tizen.org written by Adam
Malinowski (MuxPi author).

This documentation is meant to be "upstream" and the Tizen wiki article
will link to this documentation. This is why it is not linked here.

Reasons for change of documentation place:
- wiki does not provide possibility to review
- it is a good practice to keep code and documentation in one place
- it will be possible to host it on some server or use github pages
- all stack layers will have same documentation approach
- easy re-using and referencing documentation in SLAV stack

This documentation style-guide should be followed:
https://documentation-style-guide-sphinx.readthedocs.io/en/latest/style-guide.html

Change-Id: I101a09bb8013c52333f404fc6c1aac52aaab4650
Signed-off-by: Alexander Mazuruk <a.mazuruk@samsung.com>