Add FilteredLSRK54TimeStepper + upgrades to Documenter v1

[navidcy]

Closes #359

[navidcy]

@apaloczy I believe this is ready for review.

[apaloczy]

There's something wrong with the tests:

Timestepper tests: Error During Test at /var/lib/buildkite-agent/builds/gpuci-5/julialang/fourierflows-dot-jl/test/runtests.jl:218
--
MethodError: no method matching 
FilteredLSRK54TimeStepper(::FourierFlows.Equation{DataType, 
Vector{Float64}, Float64}, ::Float64, ::CPU)

I ran the tests on a local environment and they all passed, though.

[navidcy]

Yeah I'm trying to fix it. At my laptop they fail locally as well.