doc/install.texi: CRIS: Remove gone websites. Adjust CRIS targets

That is, support for cris-linux-gnu was removed in gcc-11, but
install.texi wasn't adjusted accordingly.  Also, unfortunately the
developer-related sites are gone with no replacements.  And, CRIS is
used in other chip series as well, but allude rather than list.

The generated manpages, info, pdf and html were sanity-checked.

gcc:
	* doc/install.texi <CRIS>: Remove references to removed websites and
	adjust for cris-*-elf being the only remaining toolchain.