add doxygen

hugo/content/installation/devbuild/_index.md

@@ -17,6 +17,7 @@ In particular, all contents of the `auto` directory is regenerated if out of syn
 Developers need additional tools, beyond those required for building BornAgain from source:
 - Swig, to regenerate `auto/Wrap`;
 - Ruby, which provides the embedded interpreter erb, to regenerate `auto/Examples` and `auto/MiniExamples`;
+- Doxygen, to autogenerate the API documentation;
 - clang-format, to enforce proper source formatting.
 
 Of these, clang-format is not controlled by `make`, but must be run manually.