Tutorial, Equations: Basics (#1)

* add tutorial Equations Basics

* integrate first comment from Vanille-N

* Update Tutorial_Equations_basics.v

Co-authored-by: Neven Villani <[email protected]>

* moved the fixpoint syntax from sec 2 to sec 1.1

* moved tuto

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Co-authored-by: Neven Villani <[email protected]>

Author: thomas-lamiaux