Docs: Merge error_code_list.rst and error_code_list2.rst

[kekedy0212-design]

This PR addresses Issue #20077 by merging the two separate error code documentation pages (error_code_list.rst and error_code_list2.rst) into a single, unified document. This improves the user experience by eliminating the need to guess and navigate between two different pages when looking up an error code.

Changes made:

1. Created a new unified file: docs/source/error_codes_merged.rst
2. Enhanced clarity: Added explicit **Default enabled:** Yes or **Default enabled:** No annotation to every error code entry, making this crucial information immediately visible.
3. Updated navigation: Modified docs/source/index.rst to reference the new unified file instead of the two separate ones.
4. Cleaned up: Removed the now-redundant error_code_list.rst and error_code_list2.rst files.

Benefits:

• Users can now find any error code and know its default status from a single page.
• Solves the exact problem described in the issue: “half the time I end up checking the wrong page”.
• The documentation structure shifts from a maintainer-centric split to a user-centric unified list.

Fixes #20077