docs: only try the local sibling inventory outside CI

[ocots]

Closes #950.

DocumenterInterLinks tries each InterLinks source in order and falls back on failure. The local sibling objects.inv path in docs/make.jl (kept first so a contributor with sibling repos cloned and built locally sees their own in-progress docstring/@extref changes, not the last stable release) can never resolve on CI — only this repo is checked out there — so every CI run logged 7 Failed to load inventory warnings, one per control-toolbox sibling (CTBase, CTDirect, CTFlows, CTLie, CTModels, CTParser, CTSolvers), before falling back to the remote inventory that always succeeds.

sibling_inventory(dir_name, base_url) now drops the local path entirely when ENV["CI"] == "true", so CI's tuple only ever contains sources that can actually succeed (base URL + remote inventory). Local-dev ordering and behavior are unchanged — local-first when the path exists, same fallback to the remote URL otherwise.

Verified locally, both branches of the conditional:

• CI=true julia --project=docs/ -e 'include("docs/make.jl")' → full build completes, 0 Failed to load inventory lines, no errors.
• dev-mode tuple shape unchanged (still 3 entries, local-first) when CI is unset.

No behavior change for the 4 non-sibling entries (ADNLPModels, NLPModelsIpopt, ExaModels, MadNLP, Tutorials) — those already use committed docs/inventories/*.toml files, not a sibling build path, so they were never affected.

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