[Build][Doc] Factorised the CMake-related helpers for the documentation (HTML, PDF and manual pages).
[Build][Doc] Factorised the CMake-related helpers for the documentation (HTML, PDF and manual pages).

airsim/travelccm,airsim/trademgen,airsim/airinv,airsim/airinv,airsim/avlcal,jwakely/airrac,airsim/trademgen,jwakely/airinv,jwakely/simcrs,jwakely/airinv,jwakely/simfqt,airsim/sevmgr,airsim/travelccm,jwakely/airinv,airsim/airinv,airsim/tvlsim,airsim/rmol,airsim/tvlsim,jwakely/airinv,jwakely/travelccm,jwakely/rmol,airsim/airtsp,jwakely/trademgen,jwakely/simcrs,airsim/simcrs,airsim/rmol,airsim/tvlsim,airsim/tvlsim,airsim/airinv,airsim/airinv,airsim/avlcal,airsim/tvlsim,airsim/trademgen,jwakely/simcrs,airsim/simfqt,airsim/airinv,airsim/airtsp,airsim/simlfs,jwakely/airinv,airsim/simlfs,jwakely/airrac,airsim/simfqt,airsim/rmol,airsim/tvlsim,jwakely/airrac,jwakely/simfqt,jwakely/airinv,jwakely/rmol,airsim/avlcal,airsim/tvlsim,airsim/airinv,airsim/airrac,airsim/airrac,airsim/tvlsim,airsim/sevmgr,jwakely/trademgen,jwakely/airinv,jwakely/rmol,airsim/airinv,jwakely/trademgen,airsim/sevmgr,airsim/airrac,airsim/airinv,airsim/simlfs,jwakely/simfqt,airsim/simcrs,airsim/simcrs,airsim/rmol,airsim/tvlsim,airsim/simfqt,jwakely/travelccm,jwakely/airinv,jwakely/airinv,jwakely/rmol,airsim/avlcal