docs: update for Documenter 1.x, surface missing docstrings, add POMDPs.jl page

[jamgochiana]

Four commits that bring the documentation up to date:

build(docs): update Documenter API for 1.x compat — replaces format = :html (Documenter 0.x) with format = Documenter.HTML() (Documenter 1.x). The :html form raises Cannot convert Symbol to Documenter.Writer under current Documenter.

docs: surface missing docstrings, resolve duplicate @docs, fix example links — surfaces 6 previously-undocumented public symbols (AbstractFilter, DynamicsModel, ObservationModel, unscented_transform, unscented_transform_inverse, MvNormalPDF) by adding them to the appropriate @docs blocks. Disambiguates duplicate @docs references (the bare GaussianFilters.update/predict/measure references on both kalman.md and gmphd.md were pulling in every method and producing duplicate entries — now method-signature-qualified per page). Updates Example links from notebooks/*.ipynb to examples/*.jl. docs/Project.toml gains a [sources] path entry so docs build against the in-tree source.

docs(kalman): document pomdps_updater and link POMDPs.jl examples — initial integration mention in kalman.md (later moved out by the next commit).

docs: promote POMDPs.jl integration to its own page — moves the POMDPs content out of kalman.md into a new docs/src/pomdps.md under a new Integrations section. Adds a quick-start code snippet, documents direct dispatch vs the pomdps_updater wrapper, and shows initializing a belief from an MvNormal.

Verified locally — docs build cleanly with no missing_docs or docs_block errors.