make docs more maintainable by correctly automating sidebar and contents order

[zsunberg]

can be achieved in make.jl like this:

page_order = [
    "index.md",
    "bootstrap.md",
    "beliefs.md",
    "basic.md",
    "depletion.md",
    "sampling.md",
]

makedocs(
    modules=[ParticleFilters, POMDPs],
    format=Documenter.HTML(),
    sitename="ParticleFilters.jl",
    warnonly = [:missing_docs, :cross_references],
    pages = page_order
)

and with the appropriate line in the @contents in index.md

[dylan-asmar]

I'm happy to work on this issue, but I'm not quite following the problem/proposed solution.

The pages are currently listed in the makedocs command in make.jl And @contents is used for the different sections in index.md.

Are you proposing we do something different?

[zsunberg]

I mean that if we create the page_order variable, we can re-use it in the @contents block like this:

```@contents
Pages = Main.page_order

[zsunberg]

It will be more DRY. (Don't Repeat Yourself)