Add docs on how to build in a non-default prefix
#121 (comment 1911126) shows that people do try to build librsvg and its dependencies in a non-default prefix, and this is a bit tricky. The CI scripts have this already figured out, but this is not exposed in the documentation. The compilation docs should indicate how to build librsvg and its dependencies on a different prefix.
If done before #907 (closed) and #513 (closed), the docs should also mention the need to make install
so the gdk-pixbuf loader gets installed; otherwise SVGs with <image>
elements won't work.