detabify User's Guide
The User's Guide .xml files use a fairly random-looking mix of tabs and spaces for indentation. Let's get rid of the tabs and then enforce their absence with git hooks.
Why do this? See #10425 (closed); apparently tabs within programlisting elements are rendered as some number of spaces other than 8 in the PDF version (only!) I briefly tried to find other instances of the same issue, but there are thousands of tab characters in the User's Guide and I'm too lazy to try to find only the ones within programlisting elements.