Added an option to include auto generated code in the ign_create_docs function

[ahcorde]

This PR is related with this issue gazebosim/gz-msgs#12

I have included a new argument called AUTOGENERATED_DOC to ign_create_docs. you can include a a directory of auto generated code in the Doxygen INPUT field.

For example, in the ign-msgs library:

ign_create_docs(
  API_MAINPAGE_MD "${CMAKE_BINARY_DIR}/api.md"
  TUTORIALS_MAINPAGE_MD "${CMAKE_BINARY_DIR}/tutorials.md"
  AUTOGENERATED_DOC "${CMAKE_BINARY_DIR}/include/ignition/msgs/"
  TAGFILES
    "${IGNITION-MATH_DOXYGEN_TAGFILE} = ${IGNITION-MATH_API_URL}"
)

[chapulina]

Simple and elegant ✨