Document exported API coverage

[ChrisRackauckas-Claude]

This PR should be ignored until reviewed by @ChrisRackauckas.

Summary

• Expands source docstrings for exported DiffEqGPU GPU-kernel algorithms and LinSolveGPUSplitFactorize.
• Adds a manual API page covering exported SciMLBase/DiffEqBase reexports and the lower-level LinSolveGPUSplitFactorize docs entry.
• Wires the API page into the manual pages list.

Audit

• Explicit export entries audited: 22
• public markers found in this repo: 0
• SciMLPublic/@public markers found in this repo: 0
• Missing source/original docstrings after this change: 0
• Missing rendered docs/API-page coverage after this change: 0

Verification

Commands run locally with Julia 1.10.11:

/home/crackauc/.juliaup/bin/julia +1.10 --project=. --startup-file=no -e 'using Runic; Runic.main(["--docstrings", "--check", "src/ensemblegpukernel/gpukernel_algorithms.jl", "src/ensemblegpuarray/kernels.jl"])'\n```\n\nResult: passed.\n\n```bash\n/home/crackauc/.juliaup/bin/julia +1.10 --project=. --startup-file=no -e 'using DiffEqGPU; # exported API audit script'\n```\n\nResult:\n\n```text\nexplicit_exported_count=22\npublic_marker_count=0\nscimlpublic_marker_count=0\nmissing_docstrings_count=0\nmissing_docstrings=Symbol[]\nmissing_rendered_static_count=0\nmissing_rendered_static=Symbol[]\n```\n\n```bash\ntimeout 3600 /home/crackauc/.juliaup/bin/julia +1.10 --project=docs --startup-file=no docs/make.jl\n```\n\nResult: failed before rendering with existing `[:example_block]` failures from `CUDA driver not functional` on this machine. Reproduced on clean `origin/master` at `b2c4cb4`; documented in #471. After the reexport page adjustment, no `:docs_block` error remains.\n\n```bash\ntimeout 3600 /home/crackauc/.juliaup/bin/julia +1.10 --project=. --startup-file=no -e 'using Pkg; Pkg.test()'\n```\n\nResult: failed in `GPU Kernelized Stiff ODE Mass Matrix` with `CUDA driver not functional`. Reproduced on clean `origin/master` at `b2c4cb4`; documented in #471.\n\n## Process Notes\n\n- Started from current `origin/master` and worked on a feature branch, not `master`.\n- Did not skip, silence, or weaken tests/docs.\n- Direct upstream push was denied for the authenticated account, so the PR branch is pushed from the `ChrisRackauckas-Claude` fork.