docs CI: add KLU.jl and ForwardDiff.jl to linkcheck_ignore (fix master Documentation build)

[ChrisRackauckas-Claude]

Problem

The Documentation check on master (HEAD 10df882) is failing. The build terminates with:

ERROR: LoadError: `makedocs` encountered an error [:linkcheck] -- terminating build before rendering.

The culprit is GitHub rate-limiting the CI runner's IP. linkcheck repeatedly received HTTP 429 for:

• https://github.com/JuliaSparse/KLU.jl
• https://github.com/JuliaDiff/ForwardDiff.jl

Both URLs are valid (a normal request returns 200); the 429 is transient GitHub anti-abuse throttling, not a broken link.

Fix

Add the two throttled GitHub URLs to linkcheck_ignore in docs/make.jl. This mirrors the existing, maintainer-merged convention in this file (e.g. FiniteDiff.jl / LinearSolve.jl added in #842) of ignoring GitHub links that get throttled during linkcheck.

Local verification

On Julia 1.12.6 (the "1" CI channel), parsed docs/make.jl and evaluated the literal linkcheck_ignore vector, then reproduced Documenter's ignore-matching (url == entry for string entries):

linkcheck_ignore length = 29
https://github.com/JuliaSparse/KLU.jl       ignored=true
https://github.com/JuliaDiff/ForwardDiff.jl ignored=true
https://github.com/JuliaDiff/FiniteDiff.jl  ignored=true   (pre-existing, sanity)
https://github.com/SciML/SomethingNotIgnored.jl ignored=false  (control)

So linkcheck will skip exactly these two URLs and no longer hit the 429 that terminated the build. Runic reports no formatting changes.

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Please ignore until reviewed by @ChrisRackauckas