Fix nested list indentation in docs

[yuhang-dong]

Description

Fix nested Markdown list indentation so Python-Markdown/MkDocs renders intended child lists instead of flattening them into the parent list.

This updates the affected docs sections to use 4-space indentation for nested list items, including the Web tab block in defining-your-own-catalog.md so list items stay inside the tab content.

Affected areas:

• docs/concepts/catalogs.md: Versioning Guidelines and Graceful Degradation examples
• docs/guides/renderer-development.md: Message Processing, Rendering Logic, and Client-to-Server Communication lists
• docs/guides/mcp-apps-in-a2ui.md: MCP App Standalone Sample subitems
• docs/ecosystem/a2ui-in-the-world.md: Open Source Examples descriptions
• docs/reference/agents.md: Sample Agents descriptions
• docs/guides/defining-your-own-catalog.md: Web tab list structure

Screenshots

Catalogs: Versioning Guidelines

Before, current live site:

After, local MkDocs build:

Defining Your Own Catalog: Web tab

Before, current live site:

After, local MkDocs build:

Verification

• Ran git diff --check
• Ran mkdocs build -f mkdocs.yaml
• Verified affected pages in Chrome using local mkdocs serve

Chrome DOM checks:

• Live https://a2ui.org/concepts/catalogs/#versioning-guidelines currently has Breaking Changes nested items = 0 and flat child items = 2.
• Local /concepts/catalogs/ has nested items under Breaking Changes, Non-Breaking Changes, and both Graceful Degradation examples.
• Local /guides/defining-your-own-catalog/ has the Web tab content inside the tab block, with 2 top-level list items and 1 nested item.
• Local checks also passed for /guides/renderer-development/, /guides/mcp-apps-in-a2ui/, /ecosystem/a2ui-in-the-world/, and /reference/agents/.

Note: mkdocs build --strict -f mkdocs.yaml still reports existing unrelated warnings for glossary.md nav inclusion, guides/authoring-components.md macro rendering, and two existing links in specification/v0.9-a2ui.md.

Pre-launch Checklist

• I signed the CLA.
• I read the Contributors Guide.
• I read the Style Guide.
• [] I have added updates to the CHANGELOG.
• I updated/added relevant documentation.
• My code changes (if any) have tests.
• [] If my branch is on fork, I have verified that scripts/e2e_test.sh passes.

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[gemini-code-assist]

Code Review

This pull request standardizes indentation and formatting across several documentation files, including catalogs, ecosystem guides, and renderer development references. The review feedback suggests improving consistency in the 'Defining Your Own Catalog' guide by adding a missing step to the Flutter section to match the Web section and standardizing the markdown syntax used for italicized notes.