docs: document template functions, docs validation, and create commands

[mjwolf]

Proposed commit message

This updates the extend documentation to include information on some new tooling features which have been added/updated recently.

• Complete template functions section in finishing-touches.md with all 6
  available functions (fields, event, inputDocs, url, generatedHeader,
  alertRuleTemplates) and usage examples
• Add documentation structure validation section explaining how to enable
  docs_structure_enforced in validation.yml and skip specific sections
• Update build-create-package.md with detailed wizard options for both
  elastic-package create package and create data-stream commands
• Add cross-references between documentation-guidelines.md and
  finishing-touches.md for template functions and validation

🤖 Generated with Claude Code

Co-Authored-By: Claude noreply@anthropic.com

Checklist

• I have reviewed tips for building integrations and this pull request is aligned with them.
• I have verified that all data streams collect metrics or logs.
• I have added an entry to my package's changelog.yml file.
• I have verified that Kibana version constraints are current according to guidelines.
• I have verified that any added dashboard complies with Kibana's Dashboard good practices

[github-actions]

🔍 Preview links for changed docs

• docs/extend/build-create-package.md
• docs/extend/documentation-guidelines.md
• docs/extend/finishing-touches.md

[github-actions]

Vale Linting Results

Summary: 2 suggestions found

💡 Suggestions (2)

File	Line	Rule	Message
docs/extend/build-create-package.md	51	Elastic.WordChoice	Consider using 'refer to (if it's a document), view (if it's a UI element)' instead of 'see', unless the term is in the UI.
docs/extend/finishing-touches.md	164	Elastic.WordChoice	Consider using 'refer to (if it's a document), view (if it's a UI element)' instead of 'see', unless the term is in the UI.

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[kgeller]

These look great!

[mrodm]

LGTM