Use stable PyAMG project links

[ChrisRackauckas-Claude]

Ignore this PR until it has been reviewed by @ChrisRackauckas.

Summary

• Retarget all five PyAMG references from the intermittently rate-limited Read the Docs endpoint to the official PyAMG GitHub project.
• Keep Documenter strict link checking enabled. This does not add a linkcheck ignore, warn-only setting, retry suppression, or other failure bypass.
• Keep the source docstrings, rendered manual, and sublibrary README consistent.

Baseline evidence

• Clean main commit e80aaaee failed documentation run 29722529521 with three HTTP 429 responses from https://pyamg.readthedocs.io, followed by a fatal makedocs linkcheck error: https://github.com/SciML/LinearSolve.jl/actions/runs/29722529521/job/88288359037
• PR Reuse direct BLAS LU workspaces during refactorization #1099 independently failed in the same way at 27278b6e: https://github.com/SciML/LinearSolve.jl/actions/runs/29747030285/job/88367481216
• The links were introduced with LinearSolvePyAMG in Add LinearSolvePyAMG sub-library with PyAMG integration and tests #900 (4d8c7e3b). No existing issue covers this failure.
• Add rendered public API docs QA #1096 currently contains a branch-only linkcheck_ignore workaround; this focused change preserves strict checking instead.

Validation

• Julia 1.12.4 full docs build: julia --project=docs -e 'using Pkg; Pkg.instantiate(); include("docs/make.jl")' completed with exit 0, including strict external link checking and HTML rendering.
• Runic 1.7.0 --check .: passed.
• git diff --check: passed.
• The replacement URL returned HTTP 200 for both GET and HEAD locally.