Add missing docstrings to documentation

[giordano]

I was trying in #267 to switch to Documenter.jl v1, which however turns on more strict checks and it appears there are some docstrings that we don't add to our docs:

┌ Error: 11 docstrings not included in the manual:
│ 
│     ParticleDA.get_state_indices_correlated_to_observations :: Tuple{Any}
│     ParticleDA.get_initial_state_mean!
│     ParticleDA.write_snapshot :: Union{Tuple{T}, Tuple{AbstractString, Any, NamedTuple, AbstractMatrix{T}, Int64, Bool}} where T
│     ParticleDA.write_state :: Tuple{HDF5.File, AbstractVector, Int64, String, Any}
│     ParticleDA.get_covariance_initial_state
│     ParticleDA.get_covariance_initial_state :: Tuple{Any}
│     ParticleDA.init_filter
│     ParticleDA.sample_proposal_and_compute_log_weights!
│     ParticleDA.write_observation :: Tuple{HDF5.File, AbstractVector, Int64, Any}
│     ParticleDA.write_weights :: Tuple{HDF5.File, AbstractVector, Int64, Any}
│     ParticleDA.get_initial_state_mean :: Tuple{Any}