Update docs outline generation

[dylan-asmar]

This addresses #565.

This maintains the structure as before where we have subsections for each part of then the outline. The current way @contents is structured, it only accepts a list (link to documentation) for the Pages variable. If you pass the same structure we generate for makedocs you get:

ERROR: LoadError: MethodError: Cannot convert an object of type Pair{String, Vector{String}} to an object of type String
The function convert exists, but no method is defined for this combination of argument types.

This version appends the outline to the bottom of index.md, generates the docs, and then restores the original index.md file.

With that being said...we don't have to maintain the layout as we currently have it.

[dylan-asmar]

Docs preview: https://juliapomdp.github.io/POMDPs.jl/previews/PR566