docs: add URL drilldown examples for context.panel and table row variables (#6728)

[florent-leborgne]

Summary

This PR addresses #6728 by adding practical, copy-pasteable examples to the URL drilldown documentation. All variables, helpers, trigger labels, and templates are verified against the Kibana url_drilldown implementation at HEAD.

• explore-analyze/dashboards/drilldowns.md: Add an Example URL templates section after the variables reference table, with copy-pasteable recipes for passing the panel time range, filters (rison helper), query, and data view to Discover, a full Discover deep link combining all panel context, and passing a clicked table row value to an external URL.
• explore-analyze/dashboards/drilldowns.md: Add a worked end-to-end procedure, Create a URL drilldown from a table row, that uses the Table row click trigger to pass event.values and event.keys to a URL (the most-requested scenario, which previously had no example).
• explore-analyze/dashboards/drilldowns.md: Update the Create URL drilldowns intro to acknowledge internal {{kib}} navigation and list the Table row click trigger, and correct the variables reference table heading from "Row click" to "Table row click" to match the trigger's UI label.

The full Discover deep-link template is adapted from Kibana's own functional test fixtures, so it reflects a known-working template.

Resolves

Closes #6728

Screenshots to add or update

• explore-analyze/dashboards/drilldowns.md — new Create a URL drilldown from a table row section: optionally add a screenshot of the drilldown popup on a table row click. Prose covers the steps, so this is a nice-to-have for parity with the existing pie chart and Discover examples rather than a necessity.

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🔍 Preview links for changed docs

• explore-analyze/dashboards/drilldowns.md

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