Added 'e107_docs\doxygen' folder which will contain standalone doxygen docs that aren't specific to any one e107 file

Added 'main_page.php' which contains the text which is displayed on the main page of the documentation that doxygen creates. Right now it contains the same information that's found in the current 'README.md'
Removed doxygen folder and added the markup to index.php
Merge pull request #374 from jburns131/feat/doxygen

Started working on doxygen markup