nav: add phantom for elastic-agent otel-collector slug rename - #3795

Summary

Temporary phantom to unblock CI on elastic/elastic-agent#16022, which renames reference/edot-collector/ → reference/otel-collector/ in the elastic-agent docs.

The NavigationPrefixChecker validates every page in a PR's link index against the global nav. Because the nav entry still points to edot-collector, every new otel-collector/* page fails validation. The phantom suppresses this until the nav entry is updated.

Merge sequence (all 5 PRs)

This is a coordinated 5-PR rename. All PRs must merge in order:

1. This PR — adds phantom; docs: rename reference/edot-collector slug to reference/otel-collector (Phase 4) elastic-agent#16022 CI goes green
2. docs: rename reference/edot-collector slug to reference/otel-collector (Phase 4) elastic-agent#16022 — renames reference/edot-collector/ → reference/otel-collector/ (31 pages)
3. nav: swap elastic-agent nav entry edot-collector → otel-collector, remove phantom #3799 — swaps nav entry + removes this phantom; must merge immediately after step 2 (live nav is broken in the gap between steps 2 and 3)
4. docs: rename edot-cloud-forwarder, edot-vs-upstream, and edot-collector-auth slugs (Phase 4) opentelemetry#654 + docs: rename edot-collector, quickstart, and stream-any-log-file slugs (Phase 4) docs-content#7807 — can merge simultaneously once step 3 is done

Tracked by elastic/docs-content#7801.

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