docs: Add system catalog entries for watermark and actor tables

[Copilot]

Description

Documents three new system catalog entries introduced in risingwavelabs/risingwave#15344 for querying storage table watermarks and actor information:

• rw_hummock_table_watermark - Query watermark state per table/vnode, including epoch and direction
• rw_actor_infos - Query actor metadata including node and dispatcher details
• rw_sink_decouple - View showing sink decoupling status and watermark vnode counts

All entries added in alphabetical order to the system catalog reference table.

Related code PR

risingwavelabs/risingwave#15344

Related doc issue

Fix #780

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>Document: support system table to query storage table watermark</issue_title>
<issue_description>This issue tracks the documentation update needed for the merged PR risingwavelabs/risingwave-docs#23412.

Source PR URL: risingwavelabs/risingwave#15344

If it is a major improvement that deserves a new page or a new section in the documentation, please check if we should label it as an experiment feature.</issue_description>

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

• Fixes Document: support system table to query storage table watermark #780

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[WanYixian]

LGTM！