[webdoc10] Supplement devbuild doc ()

Merging branch 'webdoc10'  into 'r21'.

See merge request !1835

hugo/content/installation/devbuild/_index.md

@@ -22,4 +22,8 @@ Developers need additional tools, beyond those required for building BornAgain f
 
 Of these, clang-format is not controlled by `make`, but must be run manually.
 
+Optionally, the following tools can be needed:
+- Graphviz, to visualize project hierarchy with doxygen;
+- hugo, to generate [web documentation](/howto/contribute-to-documentation).
+
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