Update documentation about bugs reporting and new releases notification

Update documentation about bugs reporting and new releases notification
(Cherry-pick commits e3456e4, 61fd9da from 'master' branch.)

* README.md (Download): Add link to BDWGC Download page on GitHub.
* README.md (Overview): Change link (for the further information) from
hboehm.info/gc to doc/overview.html.
* README.md (Bugs): Remove information about bugs reporting from this
section.
* README.md (Feedback, Contribution, Questions and Notifications): New
section.
* configure.ac (AC_INIT): Change URI for reporting bugs.
* doc/gc.man (SEE ALSO): Add link to the BDWGC main page on GitHub.
* doc/gcdescr.html: Remove information how to provide feedback.
* doc/overview.html (Contacts and Mailing List): Update information
(recommend use of GitHub and Stack Overflow, provide links to the
archives of the former mailing lists).