…ies (#63630)

• Warn instead of failing on missing 3rd-party doc inventories

Third-party Sphinx intersphinx inventories (e.g., Pandas) are sometimes temporarily unavailable. Previously, any download failure terminated the entire doc build. Now missing 3rd-party inventories produce warnings and fall back to cached versions when available. A marker file is written for CI to detect missing inventories and send Slack notifications on canary builds. Publishing workflows fail by default but can opt out.

• Add --fail-on-missing-third-party-inventories flag (default: off)
• Add --clean-inventory-cache flag (--clean-build no longer deletes cache)
• Cache inventories via stash action in CI and publish workflows
• Send Slack warning on canary builds when inventories are missing

• Add documentation for inventory cache handling options

Document the new --clean-inventory-cache, --fail-on-missing-third-party-inventories, and --ignore-missing-inventories flags in the contributing docs, Breeze developer tasks, and release management docs.

• Skip missing third-party inventories in intersphinx mapping

When a third-party inventory file doesn't exist in the cache, skip it from the Sphinx intersphinx_mapping instead of referencing a non-existent file. This prevents Sphinx build errors when third-party inventory downloads fail.
(cherry picked from commit afda438)

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