doc: Update docs images README with required ditaa version

doc: Update docs images README with required ditaa version

The URL for Ditaa linked to the old Sourceforge version which is
too old for what we need, the fork over on Github is the correct
version to use for re-generating the SVG files for the docs. The
required Ditaa version is 0.11.0 as it when SVG support as added.
Running the version found on Sourceforge produce the error below:

$ ditaa -E -S --svg in.txt out.txt
Unrecognized option: --svg
usage: ditaa <INPFILE> [OUTFILE] [-A] [-b <BACKGROUND>] [-d] [-E] [-e
      <ENCODING>] [-h] [--help] [-o] [-r] [-S] [-s <SCALE>] [-T] [-t
      <TABS>] [-v] [-W]

While there, also mention that meson rules exists for building
images.

Author: Nazir Bilal Yavuz <byavuz81@gmail.com>
Reviewed-by: Daniel Gustafsson <daniel@yesql.se>
Reviewed-by: Paul A Jungwirth <pj@illuminatedcomputing.com>
Discussion: https://postgr.es/m/CAN55FZ2O-23xERF2NYcvv9DM_1c9T16y6mi3vyP=O1iuXS0ASA@mail.gmail.com