cmake: Fix several bugs in installation rules
Fix a bug in Arch operators
test: Produce a diagnostic if dispatcher test is invoked incorrectly
dispatcher: Exit gracefully when NONE_NULL architecture is not linked in

If there's no SIMD extensions are supported and NONE_NULL architecture
is not provided, the only sensible thing the library can do is to
terminate the program.
doc: Add default doxygen stylesheet, header and footer

This will be customized in the future.
doc: Customize the layout templates

Set fixed width of the page and change font.
README: Move to README.md
cmake: Move the architecture support to cmake/SimdppMultiarch.cmake
Improve documentation