minify SVG images from documentation

The tarball currently distribute 5.8MB of SVG files in doc/userdoc/. Running svgo would halve that to 2.9MB.

Not included in the distribution tarball, but present on the Debian packages are also 6MB of SVG files in doc/spot.html/. Those can be reduced to 4.8MB.

Note that, at least on doc/userdoc/, svgo has to be run with --disable=removeViewBox otherwise SVG displayed with <object> are incorrectly scaled.

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information