[Alerting v2] [Create Rule Form] Add title sections for Alert conditions, Rule execution and Rule details

[ana-davydova]

Summary

Adds section titles to the v2 rule authoring flyout to match the M2 design audit.

Closes #rna-543 & closes #rna-545

Test plan

• Open Create ES|QL rule flyout (alert kind) and verify step 1 shows Alert conditions above query fields and Rule execution above alert delay / schedule / lookback
• Repeat step 1 check for signal kind — Rule execution should still appear; alert delay should not
• Open Create threshold rule flyout and verify step 1 shows the same section titles, with Data source visually smaller than Alert conditions
• On step 3 (Details & Artifacts), verify Rule details appears above name/description/tags and Artifacts remains below the horizontal rule
• Confirm create and edit flows for both rule types
• Confirm you see EuiHorizontalRule between major blocks in the flyouts (both compose discover & rule builder)
• Unit tests pass:

  node scripts/jest x-pack/platform/packages/shared/response-ops/alerting-v2-rule-form/flyout/compose_discover/compose_discover_form/compose_discover_form.test.tsx```
  
  

Threshold rule:

[kibanamachine]

💛 Build succeeded, but was flaky

• Buildkite Build
• Commit: a9455a5
• Build duration: 52 mins

Failed CI Steps

• FTR Configs #43

Metrics [docs]

Async chunks

Total size of all lazy-loaded chunks that will be downloaded as the user navigates the app

id	before	after	diff
alertingVTwo	1.2MB	1.2MB	+710.0B