Doxygen based documentation

[projectgus]

TODO:

• Add doxygen markup to existing code comments.
• Build doxygen for the project automatically to gh-pages.
• Move current wiki content to doxygen documentation pages.
• Add new content documenting OTA updates, clearer quickstart steps.

[projectgus]

Small note that some of the documentation mentioned above (like OTA update details) has been added, but is currently in the github wiki: https://github.com/SuperHouse/esp-open-rtos/wiki

Would still like eventually migrate it all to doxygen.