Buildkite: Skip PR builds when there are no doc file changes

[reakaleek]

What

• Add a conf.yaml-based path check so docs-build-pr can exit early when a product PR has no doc-related file changes
• Apply this to all repos on the default build path (including Kibana, search-ui, integration-docs, and elastic-serverless-forwarder)
• Add tests for path collection and Kibana legacy branch coverage (8.x only, no 9.x)

Why

• Some product PRs to legacy branches change only application code but still trigger a full docs build
• Legacy AsciiDoc docs are only built through 8.x, so builds for 9.x and main should skip; this change also skips when there are no doc file changes on legacy branches

Notes

• Repos with specialized path rules in build_pr.sh are unchanged
• If docs_paths.pl cannot read conf.yaml, the build still runs (fail open)

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