Add generator for modern buildsystem documentation

Summary

The buildsystem documentation generator does not support multi-line comments, rst systax and modern CMake documentation commands as used in CMake itself. This MR adds some functionality and redirects to a python module to do the rest.

added buildsystem label

mentioned in merge request !1021 (merged)

mentioned in merge request !1019 (merged)

merged

mentioned in commit 8bb933b2

mentioned in commit 1cc939ee

mentioned in merge request !1024 (merged)