[SE] Doc tweaks

Summary:
* Sections on main page.
* Use std algorithm for equality check in example.
* Add tree view on left side.
* Add extra CSS sheet to restrict content width.
* Add mild background color.
* Restrict alphabetic indexes to 1 column.
* Round corners of content boxes.
* Rename example to CUDASaxpy.cpp.
* Add CUDASaxpy.cpp to "Examples" section.

Reviewers: jprice

Subscribers: parallel_libs-commits

Differential Revision: https://reviews.llvm.org/D24198

llvm-svn: 280511