The docfx.json file points to the generated dll to be used to extract the documentation:

{
  "metadata": [
    {
      "src": [
        {
          "src": "../src",
          "files": [
            "lib/efl_mono/*efl_mono.dll"
          ]
        }
      ],

In meson builds, though, this dll is not located in the ../src folder (relative to where the docfx.json file is) but in a user-provided folder (usually build/src). Therefore, docfx.json cannot be static and has to be generated during the configure step.

Please note that generating the documentation from the source *.eo.cs files has the same problem.