lgtm.yml: disable documentation building

Don't build the documentation on LGTM as it indirectly triggers a build
of pkg/static and LGTM's env. is an Ubuntu 20.04 with nodejs 10 which
cannot build the `stylelint-webpack-plugin`. As LGTM inspects our C/C++
code we don't need to build docs so this should also speed up our CI.

lgtm.yml

@@ -3,6 +3,9 @@ extraction:
     prepare:
       packages:
         - appstream
+    configure:
+      command:  # disable documentation building as this indirectly triggers a build of pkg/static
+        - ./autogen.sh --disable-doc
     index:
       build_command:
         - make -j$(nproc) WEBPACK_PACKAGES=