Usually posix functions have individual doc pages, and each header has its own
list of required macro definitions. Use a simpler key of "in-latest-posix" to
signal that the URL convention can be followed.
Add support for a "removed-in-posix-2008" key which will link to the 2004 docs
for functions like bcmp, bcopy, bzero, index, and rindex from strings.h.
I don't want to add all of these links for pthreads.h, so automating this will
make documenting these go much faster.
bcmp, bcopy, and bzero should be moved from libc/src/string/ to
lib/src/strings/ in order for docgen to use existing conventions to find
whether we implement a function or not.
We should add support to docgen for mentioning glibc extensions (mempcpy) or
extensions from other libcs.
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.
