Documentation: Change doxygen build process

Doxygen build modifications:
    - Update doxygen requirement to 1.8.4
    - Add custom header.html with MADlib keywords and links to previous versions
    - Add custom madlib_extra.css with custom CSS
    - No colons on headers anywhere
    - Remove CSS patch and include that content in custom CSS file
    - Increase font size in TOCs
    - Enable markdown processing
    - add keywords to docs
    - increase toc.li font size
    - remove Files from topics
    - remove diagram
    - define src/images directory
- Reformat various modules to reflect doxygen changes