[Doc] add labs, rework structure

doc/a-maths/dynamics.md

doc/a-maths/geometry.md

doc/a-maths/intro.md

doc/a-maths/kinematics.md

doc/a-maths/se3.md

doc/b-usage/creating-models.md

doc/b-usage/crtp.md

doc/b-usage/intro.md

doc/c-tutorials/direct-geometry.md

doc/c-tutorials/intro.md

doc/c-tutorials/inverse-kinematics.md

doc/d-labs/1-directgeom.md

doc/d-labs/2-invgeom.md

doc/d-labs/3-invkine.md

doc/d-labs/4-dyn.md

doc/d-labs/5-planner.md

doc/d-labs/6-wpg.md

doc/d-labs/7-learn.md

doc/d-labs/intro.md

doc/treeview.dox