Fix master docs build: resolve upstream @ref cross-references (#879)

[ChrisRackauckas-Claude]

This PR should be ignored until reviewed by @ChrisRackauckas.

Fixes #879.

Problem

Master Documentation fails with makedocs encountered an error [:cross_references]. Bare @refs live inside upstream SciMLBase / OrdinaryDiffEq docstrings spliced via @docs without local targets.

Broken refs (from run 29313032312):

• problem_type, OverrideInitData, ReturnCode, reeval_internals_due_to_modification! (SciMLBase)
• Developer-Extension-API ×3 (OrdinaryDiffEq API pages; target lives under devtools/, which DiffEqDocs deletes)

Fix

1. Add @docs entries for the four SciMLBase names in the appropriate local pages.
2. Before deleting devtools/, rewrite copied [...](@ref Developer-Extension-API) links to the published OrdinaryDiffEq public_api URL.

Does not add :cross_references to warnonly.

[ChrisRackauckas-Claude]

CI note (agent): Documentation workflow has if: github.event_name == 'push' || !github.event.pull_request.draft, so the docs build was skipped while draft. Marked ready for review so Documentation can run and validate the #879 cross-ref fix.

Also rebased onto current master (includes #880 FAQ).

This PR should still be ignored until reviewed by @ChrisRackauckas.

[ChrisRackauckas]

No fix the actual bad ref

[ChrisRackauckas-Claude]

Closing in favor of source fixes rather than DiffEqDocs-side @docs padding / make.jl rewrites.

Bare @ref targets in upstream docstrings only resolve against the docs site that includes them. DiffEqDocs should not paper over that with local targets.

Upstream PRs:

• SciMLBase: Portable absolute URLs for docstring @refs that break DiffEqDocs (#879) SciMLBase.jl#1468 — absolute URLs for problem_type, OverrideInitData, ReturnCode, reeval_internals_due_to_modification! in docstrings
• OrdinaryDiffEq: Portable absolute URL for developer extension API links (DiffEqDocs #879) OrdinaryDiffEq.jl#4008 — absolute URL for Developer-Extension-API links in API pages that DiffEqDocs copies (and then deletes devtools/)

Once those land (and are registered / resolved by DiffEqDocs CI), master docs should go green without this PR.