docs: link the DiffEqDocs tutorials via stable, not latest

[ChrisRackauckas-Claude]

Please ignore until reviewed by @ChrisRackauckas.

What changed and why

The aggregate Documentation build on main is currently red:

Error: linkcheck 'https://docs.sciml.ai/DiffEqDocs/latest/getting_started/' status: 403.
Error: linkcheck 'https://docs.sciml.ai/DiffEqDocs/latest/tutorials/bvp_example/' status: 403.
ERROR: LoadError: `makedocs` encountered an error [:linkcheck] -- terminating build before rendering.

DiffEqDocs publishes stable and dev; there is no latest alias, so both URLs 403. Points them at stable (same pages, released docs). These were the only two docs.sciml.ai/<site>/latest/ links anywhere in the manual.

Verification

Checked directly with the same request Documenter's linkcheck makes (curl -sI, Chrome user agent, no redirect following), the statuses are:

https://docs.sciml.ai/DiffEqDocs/latest/getting_started/        -> 403
https://docs.sciml.ai/DiffEqDocs/latest/tutorials/bvp_example/  -> 403
https://docs.sciml.ai/DiffEqDocs/stable/getting_started/        -> 200
https://docs.sciml.ai/DiffEqDocs/stable/tutorials/bvp_example/  -> 200

and grep -rn "docs.sciml.ai/[A-Za-z]*/latest" docs/src/ now returns nothing. The linkcheck phase is the only thing that failed, so these replacements address the recorded error.

Not verified

• I did not run the full aggregate makedocs locally: it uses MultiDocumenter and clones the per-package docs repositories, which is not something I can complete here. The failing phase was linkcheck on exactly these two URLs, so CI is the real confirmation.
• Linkcheck also visits every other URL in the manual; I only checked the two that were failing.

🤖 Generated with OpenCode (model: muse-spark-1.3-contributor-free); no public session URL (local session at /home/crackauc/sandbox/tmp_20260917_040834_91074).