Adds 4 new reference pages under a new Concepts sub-parent of Reference
(cheat sheet, step type index, context variables, Liquid filters) and 1
new troubleshooting page under Authoring techniques. Enhances alert and
scheduled trigger pages. Thins context-variables tables in
pass-data-handle-errors.md and templating.md to point at the new
canonical reference.

Stacks on PR B (#6103).

Made-with: Cursor

## Summary

- Adds the `workflow-authoring` built-in skill (technical preview, gated
by `agentBuilder:experimentalFeatures`) and its 11
`platform.workflows.*` inline tools to the Agent Builder built-in skills
reference.
- Adds `platform.core.sml_search` and `platform.core.sml_attach` to the
built-in tools reference — the SML tools the skill uses to discover
existing workflows.
- Tightens descriptions of the existing
`platform.core.get_workflow_execution_status` (9.3+) and
`platform.core.resume_workflow_execution` (9.4+) tools so they link to
the Workflows landing page and the `waitForInput` step.
- Uncomments the `observability.rca` skill entry now that it's confirmed
Experimental in 9.4.

Closes elastic/docs-content-internal#1054. Stacks on the 9.4 Workflows
PRs (#6048, #6103, #6104) — forward-references `human-in-the-loop.md`
from #6048.

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Co-authored-by: Liam Thompson <leemthompo@gmail.com>

## Summary

Adds reference and troubleshooting content for Workflows 9.4 GA. Stacks
on PR B ([#6103](#6103)),
which stacks on PR A
([#6048](#6048)).

## New pages

**Under a new Concepts sub-parent of Reference:**

- \`concepts.md\` — parent page for the quick-reference section.
- \`concepts/cheat-sheet.md\` — one-page bookmark reference: anatomy,
triggers, step skeleton, step menu, Liquid patterns, context variables,
error handling, and the top gotchas.
- \`concepts/step-types.md\` — A-Z index of every step type that ships
in 9.4, with a one-line description and a deep link to its reference.
Rebuilt from PR A's shipped step pages (not the PM source index, which
predates PR A).
- \`concepts/context-variables.md\` — canonical reference for every
variable you can reference in a Liquid expression.
- \`concepts/liquid-filters.md\` — categorized filter catalog, including
the two custom filters (\`json_parse\`, \`entries\`).

**Under Authoring techniques:**

- \`authoring-techniques/troubleshooting.md\` — formal Elastic
troubleshooting content type with 18 symptom / cause / resolution
entries covering triggers, flow control, cases and alerts, Liquid, AI
steps, composition, HITL, concurrency, and setup.

## Enhancements

- \`triggers/alert-triggers.md\` — added an explicit rule-attachment
warning, a schema section, the \`event.*\` fields table, and an
alert-states table (\`new\` / \`ongoing\` / \`recovered\`).
- \`triggers/scheduled-triggers.md\` — added three callouts: the
1-minute minimum interval, the unsupported \`HOURLY\` / \`YEARLY\` rrule
frequencies, and the \`tzid\` default of UTC.

## SME review needed

**Please flag any of the following that need correction before merge.**
Each item is marked with a \`%\` comment in the relevant file.

1. **Alert trigger \`event.*\` fields.** The new payload table in
\`alert-triggers.md\` lists the top-level fields (\`event.alerts\`,
\`event.rule\`, \`event.spaceId\`, \`event.params.*\`). The individual
alert shape inside \`event.alerts\` varies by rule type (detection,
alerting, ES|QL) and source data. An SME pass on which alert fields are
guaranteed present in 9.4 GA across rule types would strengthen this
page.

2. **PM troubleshooting claims that contradict PR A.** The PM source had
four concrete schema claims that conflict with PR A's
Kibana-source-verified facts. I followed PR A in every case and flagged
for engineering confirmation:
- PM: \`workflows.failed\` \`condition\` lives under \`on:\`. **PR A:**
filters live under \`with.filters\`.
- PM: \`while\` default \`max-iterations\` is 2,000. **PR A:** no
default; unbounded unless \`max-iterations\` is set explicitly.
- PM: Resume API path is
\`/api/workflows/executions/{executionId}/resume\` and wraps the body in
\`{ \"input\": { ... } }\`. **PR A:** path is
\`/api/workflowExecutions/{executionId}/resume\` and the body is flat.
- PM: \`cases.setCustomField\` and \`cases.createCaseFromTemplate\` are
documented but "not registered" in 9.4. **PR A (verified against Kibana
Cases plugin registration):** both are registered.

3. **Cheat sheet schema claims.** Most of the cheat sheet was
cross-referenced against PR A's step reference pages during authoring.
Anything still looking off should be flagged.

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Co-authored-by: Cursor <cursoragent@cursor.com>