Doc: Relayout the download pages (issue 3595).

This slightly modifies the layout of the download pages to make
them more balanced (in the HTML version) and to accomodate wide
pictures.

For Unix, the "Specific packages" box is moved to the top right,
next to the "Generic packages" box, while the "Compiling a file"
box is now a centered, full-width box.

For MacOS and Windows, a similar change is made: the "Compiling a
file" box is moved below and turned into a centered, full-width box
to better accomodate the picture-full content.
Detect use of backslash in node names (3590)

Although a backslash in a node name appears to cause
no problems when used in a within-manual reference,
it causes cross-manual references to fail.
Doc: Remove backslash from all node names and references (3590)

  English docs only
Doc: add instructions for headings which include backslash (3590)

  and tidy up the formatting a little
Tidies removal of backslash in paper (Issue 3590)
Issue 3600: Grob `SustainPedal' has no interface for property `direction'
Build: Also track verbatiminclude'd files (issue 3594).