Variable controls in Discover

[florent-leborgne]

This PR:

• Adds a section about creating variable controls in Discover, with sub-sections on how to edit them and how to bring these controls over to a dashboard
• Transforms certain pieces of content into content snippets that are shared between Discover and Dashboards docs as variable controls are used in both apps: code examples, control parameters.
• Adds the section about "how to bring Discover sessions along with their controls to a dashboard" to the "Add controls" page in dashboards docs as well. This content is single-sourced so only maintained in one place.
• Re-adds a "preview" flag to the Variable controls section in dashboard docs, that one was mistakenly removed in another PR

Find page previews in one of the comments below

Note: Tested on staging environment on Oct 10

Closes: #3096

[Copilot]

Pull Request Overview

This PR adds comprehensive documentation for variable controls in Discover, enabling users to create dynamic queries with reusable controls that can be shared between Discover and Dashboards.

• Adds new section documenting how to create, edit, and use variable controls in Discover
• Creates shared content snippets for variable control configuration and examples to reduce duplication
• Documents the workflow for importing Discover queries with controls into dashboards

Reviewed Changes

Copilot reviewed 5 out of 7 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
explore-analyze/discover/try-esql.md	Adds complete variable controls documentation for Discover including creation, editing, and dashboard import
explore-analyze/dashboards/add-controls.md	Updates dashboard controls documentation to use shared snippets and adds import workflow
explore-analyze/_snippets/variable-control-form.md	Creates shared snippet for variable control configuration parameters
explore-analyze/_snippets/variable-control-examples.md	Creates shared snippet with ESQL query examples using variable controls
explore-analyze/_snippets/import-discover-query-controls-into-dashboard.md	Creates shared snippet documenting the import workflow from Discover to Dashboard

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[github-actions]

🔍 Preview links for changed docs

• explore-analyze/dashboards/add-controls.md
• explore-analyze/discover/try-esql.md

[stratoula]

Lovely! Added some small comments, nothing crucial

[benironside]

LGTM, left some suggestions for your consideration! Hope they're helpful