docs: add streaming_parallelism_for_backfill parameter

[Copilot]

Description

Documents streaming_parallelism_for_backfill session parameter introduced in risingwavelabs/risingwave#24026. This parameter enables separate parallelism configuration for backfill operations, with automatic transition to normal streaming parallelism after backfill completion.

Changes:

• Runtime parameters reference (operate/view-configure-runtime-parameters.mdx): Added parameter definition with usage example demonstrating parallelism switching behavior
• Backfill concepts (processing/backfill.mdx): Added parallelism control section under performance considerations with cross-reference

Example usage:

SET STREAMING_PARALLELISM = 4;
SET STREAMING_PARALLELISM_FOR_BACKFILL = 2;
CREATE MATERIALIZED VIEW mv AS SELECT * FROM large_table;
-- Uses parallelism 2 during backfill, switches to 4 after completion

Related code PR

risingwavelabs/risingwave#24026

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>Document: feat: add backfill-specific parallelism control for streaming jobs</issue_title>
<issue_description>This issue tracks the documentation update needed for the merged PR risingwavelabs/risingwave-docs#24026.

Source PR URL: risingwavelabs/risingwave#24026
Source PR Merged At: 2025-12-30T14:34:13Z

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: feat: add backfill-specific parallelism control for streaming jobs #894

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[Copilot]

Pull request overview

This PR documents the new streaming_parallelism_for_backfill session parameter introduced in risingwavelabs/risingwave#24026, which allows users to configure separate parallelism settings for backfill operations that automatically transition to normal streaming parallelism after completion.

Changes:

• Added parameter documentation in the runtime parameters reference table with clear description of behavior
• Created comprehensive usage example demonstrating the parallelism switching mechanism
• Added parallelism control section to backfill concepts page with cross-reference to detailed example

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
processing/backfill.mdx	Added "Parallelism control" subsection under performance considerations with SQL example and cross-reference link
operate/view-configure-runtime-parameters.mdx	Added parameter definition to table and created new "Usage examples" section with detailed SQL example showing parallelism transition