libctf: add CTF format specification

It's been a long time since most of this was written: it's long past
time to put it in the binutils source tree.  It's believed correct and
complete insofar as it goes: it documents format v3 (the current
version) but not the libctf API or any earlier versions.  (The
earlier versions can be read by libctf but not generated by it, and you
are highly unlikely ever to see an example of any of them.)

libctf/ChangeLog
2021-11-08  Nick Alcock  <nick.alcock@oracle.com>

	* doc/ctf-spec.texi: New file.
	* configure.ac (MAKEINFO): Add.
	(BUILD_INFO): Likewise.
	(AC_CONFIG_FILES) [doc/Makefile]: Add.
	* Makefile.am [BUILD_INFO] (SUBDIRS): Add doc/.
	* doc/Makefile.am: New file.
	* doc/Makefile.in: Likewise.
	* configure: Regenerated.
	* Makefile.in: Likewise.