This commit does a few things:
* creates libc/docs/headers/ and moves all user API related headers under it.
* updates paths and docgen
* updates the top level index to put these headers under a new "Implementation
Status" tab.
* rename some of the files to be foo.rst for foo.h (except strings, which is
currently a mix of string.h and stdlib.h)
* update the heading of some files to be in the form foo.h.
