doc: Document the features and `guix-package'.

* doc/guix.texi: Change the category to "Package management".  Add an
  @direntry for "Invoking guix-package".
  (Package Management): New chapter.

doc: Document `guix-build'.

* doc/guix.texi (Invoking guix-build): Populate.

doc: Remove unnecessary @ifinfo.

* doc/guix.texi: Remove @ifinfo around @dir{category,entry}.  Suggested
  by Karl Berry <karl@freefriends.org>.

doc: Add the stub of a manual.

* doc/guix.texi: New file.
* doc/fdl-1.3.texi: New file, copied from Gnulib.

* Makefile.am (info_TEXINFOS): New variable.
  (EXTRA_DIST): Add `doc/fdl-1.3.texi'.