qdoc: Add a mechanism for defining redirects in .qdoc syntax

Add a qdoc command for saving 'redirect from' information in page metadata. This metadata could be used for automating the generation of redirects (301 code) for pages where the .html filename changed.

This would be helpful for

• More stable links, avoiding 404's . In fact, broken links might result in the page / website being ranked down in search engines
• Having 'canonical' URL's that one can use e.g. in the context help

Proposed syntax

\meta redirect-from foo.html