Weird. A few hours ago, the URL of the image was https://docs.softwareheritage.org/devel/swh-model/swh-model/images/swh-merkle-dag.svg (which does not and did not exist)
but now it's https://docs.softwareheritage.org/devel/_images/swh-merkle-dag.svg (which exists)

As the image here: https://docs.softwareheritage.org/devel/_images/ was touched two minutes after I sent this Diff, it looks like my Diff triggered a rebuild of the website.

There is no CI building the doc for now (which is a problem per se, but that's a different debate :-)) However I did publish the doc myself ~1 hour ago.
I think what happened is that the last person who published the doc (I think @seirl ) didn't build the images locally and published that way, inducing the error you were seeing.
This is certainly proof that our publishing process for the doc is too flaky.
But on the other hand I don't think this diff is needed — the problem is elsewhere, e.g., doc build should fail rather than publish broken docs.

When building on my computer, I had the same issue with the missing image, before that patch.

can you tell me how did you build?

In D451#8735, @vlorentz wrote:

~/swh-environment/swh-docs/docs
make

well that's what I use too, and it works for me (without your patch)… so we need to debug what's the difference here :)

Try this before running make:

rm ../../swh-model/docs/_build/html/_images/swh-merkle-dag.svg
rm ../../swh-model/docs/images/swh-merkle-dag.svg
make clean

Gotcha!

So, the problem is that running "make" into swh-model/docs/ should also run "make" into swh-model/docs/images/, which currently isn't done. (And, symmetrically, make clean should also clean under images.)

That would be the right fix here.