Add an Override Class Index

This page pulls in directives that declare an <override> element and
groups them by class. The documentation for each class comes from
overrides.xml. Any undocumented classes are flagged with a boilerplate
fallback text (this situation is only likely to occur with a misspelling
or otherwise invalid <override> somewhere else in the documentation).

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/trunk-override-index@1793922 13f79535-47bb-0310-9956-ffa450edef68