add table of contents sections

Author: mohamed82008

docs/src/gradients/gradients.md

@@ -13,6 +13,8 @@ In some cases, function modifiers can even be composed on top of each other to c
 
 > In `Nonconvex`, function modifiers modify the behaviour of a function when differentiated once or twice using either `ForwardDiff` or any [`ChainRules`](https://github.com/JuliaDiff/ChainRules.jl)-compatible AD package, such as `Zygote.jl`. The following features are all implemented in [`NonconvexUtils.jl`](https://github.com/JuliaNonconvex/NonconvexUtils.jl) and re-exported from `Nonconvex`.
 
+## Table of contents
+
 ```@contents
 Pages = ["user_defined.md", "other_ad.md", "chainrules_fd.md", "sparse.md", "symbolic.md", "implicit.md", "history.md"]
 Depth = 3

docs/src/problem/problem.md

@@ -5,6 +5,8 @@ There are 3 ways to define a model in Nonconvex.jl:
 2. `DictModel` which assumes each variable has a name. The decision variables are stored in an `OrderedDict`, an ordered dictionary data structure.
 3. Start from `JuMP.Model` and convert it to `DictModel`. This is convenient to make use of `JuMP`'s user-friendly macros for variable and linear expression, objective or constraint definitions.
 
+## Table of contents
+
 ```@contents
 Pages = ["model.md", "dict_model.md", "queries.md"]
 Depth = 3