Write documentation for phoenix2 library

API/code documentation is generated by doxygen. What is missing is:

• "What does this package, get started" README-like documentation
• "User guide" documentation: more in-depths usage, examples, benefits wrt competitors, etc.
• "Developer guide" documentation: library implementation choices, design, integrated with API/code documentation + how to contribute.
• Integrated documentation of all phoenix packages for the whole library documentation.

Story list:

• #18 (closed) - Documentation tools choice
• #19 - Phoenix2 Packages documentation