feat: make informal_definition and informal_lemma commands (#300)

* make informal_definition and informal_lemma commands
* drop the fields "math", "physics", and "proof" from InformalDefinition/InformalLemma and use docstrings instead
* render informal docstring in dependency graph

HepLean/Tensors/TensorSpecies/Contractions/Basic.lean

@@ -48,17 +48,19 @@ lemma contractSelfField_equivariant {S : TensorSpecies} {c : S.C} {g : S.G}
   simpa using congrFun (congrArg (fun x => x.hom.toFun)
     ((S.contractSelfHom c).comm g)) (ψ ⊗ₜ[S.k] φ)
 
-informal_lemma contractSelfField_non_degenerate where
-  math :≈ "The contraction of two vectors of the same color is non-degenerate.
-    I.e. ⟪ψ, φ⟫ₜₛ = 0 for all φ implies ψ = 0."
-  proof :≈ "The basic idea is that being degenerate contradicts the assumption of having a unit
-    in the tensor species."
-  deps :≈ [``contractSelfField]
+/-- The contraction of two vectors of the same color is non-degenerate, i.e., `⟪ψ, φ⟫ₜₛ = 0` for all
+`φ` implies `ψ = 0`.
 
+Proof: the basic idea is that being degenerate contradicts the assumption of having a
+unit in the tensor species.
+-/
+informal_lemma contractSelfField_non_degenerate where
+  deps := [``contractSelfField]
+
+/-- The contraction `⟪ψ, φ⟫ₜₛ` is related to the tensor tree
+`{ψ | μ ⊗ (S.dualRepIsoDiscrete c).hom φ | μ}ᵀ`. -/
 informal_lemma contractSelfField_tensorTree where
-  math :≈ "The contraction ⟪ψ, φ⟫ₜₛ is related to the tensor tree
-    {ψ | μ ⊗ (S.dualRepIsoDiscrete c).hom φ | μ}ᵀ "
-  deps :≈ [``contractSelfField, ``TensorTree]
+  deps := [``contractSelfField, ``TensorTree]
 
 /-!