docs: document graphviz requirement to build docs

Fixes #10636

ci/run-docs

@@ -6,5 +6,12 @@
 
 set -e
 
+if ! command -v dot &> /dev/null
+then
+    echo "dot command could not be found, please install graphviz"
+    exit 1
+fi
+
+
 # Build documentation with warnings treated as errors
 make -C docs "${1:-html}" SPHINXOPTS='-n -W -a --keep-going'

docs/contributing/index.rst

@@ -52,6 +52,10 @@ Please respect these guidelines while writing:
     1. Rewrite
     2. Once the rewrite is reviewed, polished, and merged, change the structure of the paragraphs in another PR.
 
+Documentation can be also edited and built locally, the Python requirements are
+in `docs/requirements.txt`. You will also need :program:`graphviz` to build the
+documentation.
+
 .. hint::
 
     You can `translate the docs <https://hosted.weblate.org/projects/weblate/documentation/>`_.