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