doc: Fix implement-c.texi menu

doc: Fix implement-c.texi menu

commit fbb7f1cb5d3c8b7604e8f966e04330570de3e3ca
Author: Joseph Myers <josmyers@redhat.com>
Date:   Thu May 22 21:39:37 2025 +0000

    c: Document C23 implementation-defined behavior

broke bootstrap on Ubuntu 20.04 (which bundles makeinfo 6.7)
and elsewhere with a local installation of makeinfo 6.6:

gcc/doc/implement-c.texi:5: node `C Implementation' lacks menu item for
`Constant expressions implementation' despite being its Up target
gcc/doc/implement-c.texi:5: node `C Implementation' lacks menu item for
`Types implementation' despite being its Up target

Fixed by adding the missing menu entries.

2025-05-23  Rainer Orth  <ro@CeBiTec.Uni-Bielefeld.DE>

	gcc:
	* doc/implement-c.texi (C Implementation): Add missing menu items.