[WIP] Improve CREATE SUBSCRIPTION documentation coverage

[Copilot]

Plan for CREATE SUBSCRIPTION Documentation

• Create sql/commands/sql-create-subscription.mdx with comprehensive documentation
  • Add frontmatter with title and description
  • Add complete syntax section
  • Add parameters/clauses section with detailed descriptions
  • Add behavior notes section (retention, snapshot semantics, supported sources)
  • Add minimal runnable example
  • Add realistic production example
  • Add cross-links to related pages
• Create sql/commands/sql-drop-subscription.mdx for completeness
  • Add frontmatter
  • Add syntax and parameters
  • Add examples
  • Add cross-links
• Update docs.json to add new pages to navigation
  • Add sql-create-subscription after sql-create-source
  • Add sql-drop-subscription in appropriate DROP section
• Update related pages with cross-links
  • Update serve/subscription.mdx to reference new command page
  • Update sql-alter-subscription.mdx with cross-link
  • Update sql-show-subscriptions.mdx with cross-link
  • Update sql-show-subscription-cursors.mdx with cross-link
• Verify navigation and search index visibility

Original prompt

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This section details on the original issue you should resolve

<issue_title>Improve CREATE SUBSCRIPTION documentation coverage [jarvis-v2-smoke-2026-02-17]</issue_title>
<issue_description>## Context

• Slack transcript (2026-02-17): [2026-02-17T00:00:00.000Z] <@U123TEST>: /docs smoke test please file an issue for improving CREATE SUBSCRIPTION docs and include marker [jarvis-v2-smoke-2026-02-17]
• Smoke marker: [jarvis-v2-smoke-2026-02-17]
• Related docs references from context search:
  • serve/subscription.mdx
  • sql/commands/sql-alter-subscription.mdx
  • sql/commands/sql-show-subscription-cursors.mdx
  • sql/commands/sql-show-subscriptions.mdx
• Related code/test references from context search:
  • e2e_test/ddl/subscription.slt
  • src/sqlparser/src/ast/statement.rs

Problem

The docs currently lack a clear, command-level reference for CREATE SUBSCRIPTION in the SQL command docs. Subscription information exists, but users looking for a canonical SQL syntax page can miss key details (full syntax, options, behavior notes, and runnable examples).

Suggested Fix

1. Add a dedicated SQL command reference page for CREATE SUBSCRIPTION (for example, sql/commands/sql-create-subscription.mdx).
2. Include complete syntax, parameter definitions, and behavior notes (startup position/initial snapshot semantics, supported sources, constraints).
3. Add at least one minimal runnable example and one realistic production-style example; align with existing subscription e2e coverage.
4. Cross-link the new page with serve/subscription.mdx, ALTER SUBSCRIPTION, SHOW SUBSCRIPTIONS, and SHOW SUBSCRIPTION CURSORS pages to improve discoverability.
5. Verify navigation/search index visibility so CREATE SUBSCRIPTION is directly discoverable from SQL commands docs.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Improve CREATE SUBSCRIPTION documentation coverage [jarvis-v2-smoke-2026-02-17] #991

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