Fix typo in GC overview file

* doc/overview.html: Fix a typo ("See").
Fix or remove broken URLs in documentation

* doc/README.cords: Remove broken link to reality.sgi.com.
* doc/README.environment: Fix link to HPL tech reports.
* doc/overview.html: Remove broken link to parcftp.xerox.com.
* doc/overview.html: Fix (or update) external links.
Update emails/links due to project site and ML transition
(www.hpl.hp.com/personal/Hans_Boehm -> www.hboehm.info;
gc@linux.hpl.hp.com -> bdwgc@lists.opendylan.org)

* AUTHORS: Update email.
* doc/gc.man: Likewise.
* README.QUICK: Change external link to gcinterface.html to local one
(pointing to the file in "doc" folder).
* README.md: Replace BDWGC site URL (www.hpl.hp.com/personal/Hans_Boehm
to www.hboehm.info).
* doc/gc.man: Likewise.
* doc/gcdescr.html: Likewise.
* doc/gcinterface.html: Likewise.
* doc/overview.html: Likewise.
* doc/scale.html: Likewise.
* README.md: Update mailing list info (gc@linux.hpl.hp.com to
bdwgc@lists.opendylan.org, gc-announce@linux.hpl.hp.com to
bdwgc-announce@lists.opendylan.org).
* configure.ac: Likewise.
* doc/gcdescr.html: Likewise.
* doc/overview.html: Likewise.
* doc/tree.html: Remove unnecessary external link.