CI: scheduled no-gpl build failed because of missing @refs

[inkydragon]

build error

[ Info: CrossReferences: building cross-references.
┌ Error: Cannot resolve @ref for md"[`cholesky`](@ref SparseArrays.CHOLMOD.cholesky)" in src/stdlib/SparseArrays.md.
│ - Exception trying to find docref for `SparseArrays.CHOLMOD.cholesky`: unable to get the binding for `SparseArrays.CHOLMOD.cholesky` in module Main
└ @ Documenter /Users/julia/.julia/scratchspaces/a66863c6-20e8-4ff4-8a62-49f30b1f605e/agent-cache/default-honeycrisp-XG3Q6T6R70.0/build/default-honeycrisp-XG3Q6T6R70-0/julialang/julia-master-scheduled/doc/deps/packages/Documenter/pA5Sa/src/utilities/utilities.jl:44
┌ Error: Cannot resolve @ref for md"[`ldlt`](@ref SparseArrays.CHOLMOD.ldlt)" in src/stdlib/SparseArrays.md.
│ - Exception trying to find docref for `SparseArrays.CHOLMOD.ldlt`: unable to get the binding for `SparseArrays.CHOLMOD.ldlt` in module Main
└ @ Documenter /Users/julia/.julia/scratchspaces/a66863c6-20e8-4ff4-8a62-49f30b1f605e/agent-cache/default-honeycrisp-XG3Q6T6R70.0/build/default-honeycrisp-XG3Q6T6R70-0/julialang/julia-master-scheduled/doc/deps/packages/Documenter/pA5Sa/src/utilities/utilities.jl:44
┌ Error: Cannot resolve @ref for md"[`lu`](@ref SparseArrays.UMFPACK.lu)" in src/stdlib/SparseArrays.md.
│ - Exception trying to find docref for `SparseArrays.UMFPACK.lu`: unable to get the binding for `SparseArrays.UMFPACK.lu` in module Main
└ @ Documenter /Users/julia/.julia/scratchspaces/a66863c6-20e8-4ff4-8a62-49f30b1f605e/agent-cache/default-honeycrisp-XG3Q6T6R70.0/build/default-honeycrisp-XG3Q6T6R70-0/julialang/julia-master-scheduled/doc/deps/packages/Documenter/pA5Sa/src/utilities/utilities.jl:44
┌ Error: Cannot resolve @ref for md"[`qr`](@ref SparseArrays.SPQR.qr)" in src/stdlib/SparseArrays.md.
│ - Exception trying to find docref for `SparseArrays.SPQR.qr`: unable to get the binding for `SparseArrays.SPQR.qr` in module Main
└ @ Documenter /Users/julia/.julia/scratchspaces/a66863c6-20e8-4ff4-8a62-49f30b1f605e/agent-cache/default-honeycrisp-XG3Q6T6R70.0/build/default-honeycrisp-XG3Q6T6R70-0/julialang/julia-master-scheduled/doc/deps/packages/Documenter/pA5Sa/src/utilities/utilities.jl:44
[ Info: CheckDocument: running document checks.
[ Info: Populate: populating indices.
ERROR: LoadError: `makedocs` encountered an error [:cross_references] -- terminating build before rendering.

https://buildkite.com/julialang/julia-master-scheduled/builds/924#0192bd8b-9277-4dbe-86cb-f5907d12e5e8/1859-1945

related docs

SparseArrays.jl docs:

1. [`cholesky`](@ref SparseArrays.CHOLMOD.cholesky)
2. [`ldlt`](@ref SparseArrays.CHOLMOD.ldlt)
3. [`lu`](@ref SparseArrays.UMFPACK.lu)
4. [`qr`](@ref SparseArrays.SPQR.qr)

https://github.com/JuliaSparse/SparseArrays.jl/blob/485fd4babbeaed76906e88b6adc97496346727de/docs/src/index.md?plain=1#L209-L222

All of these solvers are GPL-licensed, and SparseArrays will not include their definitions when USE_GPL_LIBS:=0.

Perhaps the easiest way to fix this is to move those lines to docs/solvers.md,
since julia only copies docs/index.md when building documentation.

[DilumAluthge]

Perhaps the easiest way to fix this is to move those lines to docs/solvers.md, since julia only copies docs/index.md when building documentation.

Seems reasonable to me. Can you make a PR?

[ViralBShah]

I assume we need to bump SparseArrays here now to fix this.

[inkydragon]

Now, all non-GPL build looks good.

https://buildkite.com/julialang/julia-master-scheduled/builds/960