Fix H1 header alignment: change from center to left justification

[djvolz]

Type of changes

• Bug fix
• New feature
• Documentation / docstrings
• Tests
• Other

Checklist

• I've run the latest black with default args on new code.
• I've updated CHANGELOG.md and CONTRIBUTORS.md where appropriate.
• I've added tests for new code.
• I accept that @willmcgugan may be pedantic in the code review.

Description

This PR fixes the alignment of H1 headers in markdown rendering by changing them from center-justified to left-justified. This improves readability, especially in terminal contexts where left-aligned text is generally easier to scan and read.

Changes Made

• Modified rich/markdown.py line 147: Changed text.justify = "center" to text.justify = "left" in the Heading class's __rich_console__ method
• Updated CHANGELOG.md: Added entry for this behavior change under [Unreleased] section
• Updated CONTRIBUTORS.md: Added contributor entry alphabetically by surname
• Updated test expectations: Modified both test_markdown.py and test_markdown_no_hyperlinks.py to expect left-aligned H1 headers

Before/After Comparison

Before (center-justified):

┏━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━┓
┃                        My Header Title                         ┃
┗━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━┛

After (left-justified):

┏━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━┓
┃ My Header Title                                                ┃
┗━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━┛

Impact

• Scope: This change only affects H1 headers (which are rendered inside panels with borders)
• Other headers: H2-H6 headers remain unchanged as they were already left-aligned
• Readability: The change improves readability and consistency with standard markdown conventions
• Breaking change: Tests expecting center-aligned H1 headers have been updated to match the new behavior

Testing

• ✅ Verified the change works by testing with python -m rich.markdown
• ✅ Updated test expectations in both test_markdown.py and test_markdown_no_hyperlinks.py
• ✅ Ran black formatter with no changes needed
• ✅ All markdown-related tests should now pass with the updated expectations

Motivation

This addresses the community feedback about centered headers being less readable in terminal environments. Left-aligned headers are more consistent with typical markdown rendering and improve the overall user experience when reading documentation in the terminal.

Fixes #2137

Additional Context

The original issue highlighted that centered headers can be harder to read, especially when scanning through documentation. This small change brings Rich's markdown rendering more in line with common expectations while maintaining the distinctive panel styling for H1 headers.