Correctly handle internal page section references.

* site.js (load_content) ["#" in URL]: Scroll to internal reference.
* site.css (overlapped): New styling class; define it, such that any
element to which it is assigned will be positioned, with its topmost
content fully visible below the page header block, when an internal
page reference is scrolled into view.

* contact.html (general-enquiry, bug-report, feature-request)
* mailing.html (mingw-users, mingw-notify, mingw-msys, list-etiquette)
* about.html (cross-compile): Mark reference points as "overlapped".