add experimental folder to doc builder function
add experimental folder to doc builder function


Former-commit-id: 313153a04c493a0fbd5f95671b19f4c61a78305f [formerly 9bbecd0a17a15e777f4bd4c8b842d33430a2502a]
Former-commit-id: 1d1443f73737b5ad2dbb992975c76cd29aed7afc
gem/oq-engine,gem/oq-engine,gem/oq-engine,gem/oq-engine,gem/oq-engine