Improve CREATE SUBSCRIPTION documentation coverage [jarvis-v2-smoke-2026-02-17]

[kwannoel]

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.