Add CREATE SUBSCRIPTION and DROP SUBSCRIPTION DDL command documentation

[Copilot]

Description

CREATE SUBSCRIPTION and DROP SUBSCRIPTION commands were undiscoverable in the DDL commands section despite existing documentation on the Subscription page.

Added dedicated DDL command pages for both commands with complete syntax, parameters, and examples. Integrated into navigation:

• Created sql/commands/sql-create-subscription.mdx - syntax, parameter table, examples
• Created sql/commands/sql-drop-subscription.mdx - syntax, parameter table, examples
• Updated sql/commands/overview.mdx - added card entries in alphabetical order
• Updated docs.json - added navigation entries

Both pages link to /serve/subscription for comprehensive subscription documentation (cursors, fetching, management).

Related code PR

N/A - documentation only

Related doc issue

https://github.com/risingwavelabs/risingwave-docs/issues/[issue-number]

Checklist

• I have run the documentation build locally to verify the updates are applied correctly.
• For new pages, I have updated mint.json to include the page in the table of contents.
• All links and references have been checked and are not broken.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Add DDL command link for CREATE SUBSCRIPTION to Subscription docs</issue_title>
<issue_description>## Context
Slack thread: https://slack.com/archives/C034XCYJPSN/p1770630289319789

Problem

Users looking for the basic "CREATE SUBSCRIPTION" command in the DDL command section cannot find it. The information exists on the Subscription page, but it is not linked from the DDL command area, making it hard to discover.

Suggested Fix

Add a DDL command entry/link for "CREATE SUBSCRIPTION" that points directly to the existing section on the Subscription page: https://docs.risingwave.com/serve/subscription#create-subscription. If there is a DDL commands index page, add the link there as well and ensure the label matches other DDL command entries.

Existing Documentation

- https://github.com/risingwavelabs/risingwave-docs/blob/f2c9267a4f86e94164809db7374cda7da217fffb/serve/subscription.mdx - https://github.com/risingwavelabs/risingwave-docs/blob/f2c9267a4f86e94164809db7374cda7da217fffb/sql/commands/sql-alter-subscription.mdx - https://github.com/risingwavelabs/risingwave-docs/blob/f2c9267a4f86e94164809db7374cda7da217fffb/sql/commands/sql-show-subscriptions.mdx - https://github.com/risingwavelabs/risingwave-docs/blob/f2c9267a4f86e94164809db7374cda7da217fffb/sql/commands/sql-show-subscription-cursors.mdx - https://github.com/risingwavelabs/risingwave-docs/blob/f2c9267a4f86e94164809db7374cda7da217fffb/sql/commands/sql-alter-swap.mdx - https://github.com/risingwavelabs/risingwave-docs/blob/f2c9267a4f86e94164809db7374cda7da217fffb/operate/workload-isolation-interaction.mdx

Codebase References

- https://github.com/risingwavelabs/risingwave/pull/24706 - https://github.com/risingwavelabs/risingwave/pull/24297 - https://github.com/risingwavelabs/risingwave/pull/24316 - https://github.com/risingwavelabs/risingwave/pull/24318 - https://github.com/risingwavelabs/risingwave/pull/19967 - https://github.com/risingwavelabs/risingwave/blob/dbc4229c844427121cd194fe35a4290c77f5ade5/src/sqlparser/src/ast/mod.rs - https://github.com/risingwavelabs/risingwave/blob/dbc4229c844427121cd194fe35a4290c77f5ade5/src/sqlparser/src/ast/statement.rs - https://github.com/risingwavelabs/risingwave/blob/dbc4229c844427121cd194fe35a4290c77f5ade5/integration_tests/client-library/java/src/test/java/com/risingwave/TestSubscription.java - https://github.com/risingwavelabs/risingwave/blob/dbc4229c844427121cd194fe35a4290c77f5ade5/src/tests/regress/data/sql/object_address.sql - https://github.com/risingwavelabs/risingwave/blob/dbc4229c844427121cd194fe35a4290c77f5ade5/src/tests/regress/data/expected/object_address.out

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

• Fixes Add DDL command link for CREATE SUBSCRIPTION to Subscription docs #953

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