docs/: ignore gtester Makefile targets

docs/: ignore gtester Makefile targets

Mark 'test', 'test-report', 'perf-report' and 'full-report' as PHONY in
docs/Makefile.am to prevent recursion of gtester into the documentation
subdirectories.  Stop including Makefile.decl from these directories
since it is no longer necessary.

This will clear up the warnings about EXTRA_DIST being defined once in
gtk-doc.make and again in Makefile.decl.