Generalized update-gh-pages even more

* Creating a skeleton gh-pages when not present.
    * Default index.html with DocumentUp submodule
    * styles.css for small fix to DocumentUp's style and for site-specific styles.
* Cleaned up remaining 3X-specific codes.
    * Separated some page manipulation as insert-nav-extras.
    * GitHub repo name derivation from remote URL.
* Handling all markdown documents (.md, .mkd, .markdown) instead of just
  README.md.
compile-xdocs for compiling executable documents