docs: document ES|QL Data Federation datasets in Kibana (#7633)

[florent-leborgne]

Summary

This PR addresses #7633 with the following changes:

• explore-analyze/query-filter/languages/esql-kibana.md: extends the FROM source-command list to note it can also target ES|QL Data Federation datasets, and adds a tip explaining that Discover and Dashboards now open even without local or remote indices when at least one dataset exists (Kibana PR #279421).
• explore-analyze/discover/try-esql.md: adds External data to the list of source types shown in the ES|QL data source browser, a pre-existing gap the issue author flagged (Kibana PR #276193, shipped in 9.5.0, not tied to the triggering PR).

Notes for reviewer

• ES|QL Data Federation itself is stack: experimental =9.5 / serverless: unavailable on its canonical reference page (elastic/elasticsearch), a very recently reviewed and deliberately per-minor-pinned tag (not an open-ended "9.5+"). I mirrored that same exact-version pinning convention on all three new facts here (=9.5 for the two 9.5-era Kibana capabilities, =9.6 for the Discover/Dashboards no-data-screen bypass, which is Kibana-only and targets v9.6.0, not yet released). A fresh-reader comprehension test initially caught a version-notation contradiction between an open-ended tag here and the source page's exact pin; aligning both resolved it.
• The PR also adds a 5-second timeout fallback (opens an empty ES|QL results table instead of failing) and parallelizes data-source checks for speed. Neither adds a user-facing option or step, so I left them out of the docs per the whether-to-document gate.
• I did not document the "browse the data source badge" / autocomplete resource-browser mechanics beyond the type-list addition; those were already covered and unaffected by this change.

Companion PR

• elastic/elasticsearch#156198 — adds a reciprocal link from the ES|QL Data Federation overview page back to "Use ES|QL in the Kibana UI", per this repo's bidirectional cross-discoverability convention.

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Review all generated content for factual accuracy before merging.

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🔍 Preview links for changed docs

• explore-analyze/discover/try-esql.md
• explore-analyze/query-filter/languages/esql-kibana.md

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