doc: Adding {D}AHC documentation

This commit add the rst documentation for AHC & DAHC.

This includes the description of why benchmarking infrastructures are
important, what are the concepts of AHC and how to use it.
health-server: runtime is a generic parameter

Getting the runtime is a generic option of jobs, so let's use it in
parse_jobs() instead of getting it computed by all benchmarks tests.
doc: Adding {D}AHC documentation

This commit add the rst documentation for AHC & DAHC.

This includes the description of why benchmarking infrastructures are
important, what are the concepts of AHC and how to use it.