cubicweb-mercurial-server #2752072 doc: write a minimal amount [validation pending]

there is none right now

prioritynormal
typebug
done in0.2.1
load0.200
load left0.000
closed by#739012fa39b9 doc: write a minimal amount (for the experienced mercurial-server and cubicweb practitioner) (closes #2752072)
patchdoc: write a minimal amount (for the experienced mercurial-server and cubicweb practitioner) (closes #2752072) [applied]