Add [compat] entry for Documenter

[MilesCranmer]

Documenter.jl introduces a number of breaking changes which seems like they weren't accounted for yet. This fixes the [compat] settings; we can upgrade later.

[github-actions]

Benchmark Results

	master	3a2be26...	t[master]/t[3a2be26...]
search/multithreading	0.0198 ± 0.0019 h	0.0198 ± 0.0019 h	0.997
search/serial	0.0188 ± 0.00014 h	0.0198 ± 0.00034 h	0.953
time_to_load	3.13 ± 0.25 s	2.98 ± 0.032 s	1.05
utils/best_of_sample	1.6 ± 0.6 μs	1.5 ± 0.6 μs	1.07
utils/check_constraints_x10	23.8 ± 6.2 μs	23.8 ± 6.2 μs	1
utils/compute_complexity_x10/Float64	4 ± 0.2 μs	3.7 ± 0.2 μs	1.08
utils/compute_complexity_x10/Int64	4 ± 0.2 μs	4 ± 0.2 μs	1
utils/compute_complexity_x10/nothing	3 ± 0.2 μs	3 ± 0.2 μs	1
utils/optimize_constants_x10	0.059 ± 0.012 s	0.0566 ± 0.012 s	1.04

Benchmark Plots

A plot of the benchmark results have been uploaded as an artifact to the workflow run for this PR.
Go to "Actions"->"Benchmark a pull request"->[the most recent run]->"Artifacts" (at the bottom).