Add git-diff exporter for CI changed-pages.json

[reakaleek]

Why

• Preview comment jobs guess page URLs from GitHub changed-file paths. Those URLs do not match navigation URLs, path prefixes, or pages that include changed snippets.
• CI needs a builder-produced list of published page URLs and titles.

What

• Add an opt-in gitdiff exporter that writes changed-pages.json during isolated builds.
• Map git changes (or GitHub Actions file lists) onto built pages and include reverse-index entries.
• Auto-enable the exporter on CI isolated builds.

Notes

• Merge the matching elastic/docs-actions PR before preview comments use the new artifact.
• Git failures are soft. The build writes an empty pages array and logs a warning.

Made with Cursor

[Mpdreamz]

One blocker before this is ready: GitChangedFileSource.ResolveDiffBase returns origin/<GITHUB_BASE_REF> immediately without checking that the ref exists. On pull_request CI with a shallow merge checkout, GITHUB_BASE_REF is set but origin/main is not necessarily present, so git diff origin/main HEAD can fail and the exporter will write an empty changed-pages.json instead of falling back to HEAD^1. The docs and tests say named bases fall back when missing, but that path is skipped for GITHUB_BASE_REF. Please verify the GitHub base ref before returning it, or make the diff step fall back when that ref is unusable.