manual: symlink manual/images/ subdir for out-of-source builds

manual: symlink manual/images/ subdir for out-of-source builds

Execution of "make manual" created HTML output, but document inspection
suffered from missing screenshots. Symlink the images/ subdirectory so
that source file content is re-used in output hierarchies.

This has gone unnoticed because "make install" references source files,
and the manual author appears to not use out of source builds. Only
out of source builds and only HTML output inspection before installation
were affected. Installed manuals were not affected.

How to reproduce:

  $ cmake --build $OUT --target manual
  $ xdg-open $OUT/manual/manual.html