Remove Pathfinder code example from docs

[penelopeysm]

Pathfinder.jl is lovely, but we probably shouldn't have a code example in there:

docs/tutorials/docs-16-using-turing-external-samplers/index.qmd

Lines 82 to 99 in 2260d61

	```{julia}
	using AdvancedHMC, Pathfinder
	# Running pathfinder
	draws = 1_000
	result_multi = multipathfinder(model, draws; nruns=8)
	# Estimating the metric
	inv_metric = result_multi.pathfinder_results[1].fit_distribution.Σ
	metric = DenseEuclideanMetric(Matrix(inv_metric))
	# Creating an AdvancedHMC NUTS sampler with the custom metric.
	n_adapts = 1000 # Number of adaptation steps
	tap = 0.9 # Large target acceptance probability to deal with the funnel structure of the posterior
	nuts = AdvancedHMC.NUTS(tap; metric=metric)
	# Sample
	chain = sample(model, externalsampler(nuts), 10_000; n_adapts=1_000)
	```

Rationale:

1. We want the docs to use the most recent version of Turing.jl, as far as is possible. Pathfinder is the only reverse dep of Turing that we are using*, meaning that if we release a new version of Turing, we need Pathfinder to also release a new version that is compatible with it. We shouldn't expect Pathfinder's maintainers to do this (they may not want to, for whatever reason).

2. We shouldn't be responsible for documenting how another package is used. It's great to point out other packages in the Julia ecosystem that can be used together with Turing, but the question of how to use package X should be documented in package X. Indeed, Pathfinder's docs already contain an example that is remarkably similar to our own: https://mlcolab.github.io/Pathfinder.jl/stable/examples/turing/

* There's also TuringBenchmarking, but that's internal, so we can easily tag a new release whenever we need to.

[penelopeysm]

Closed by #579