docs: remove stale AssertionGroup documentation

[t0k0shi]

The AssertionGroup class was removed in a previous refactor, but the documentation page and sidebar entry remained. This removes the orphaned
docs page and its sidebar reference.

Description

Removed the stale assertion-groups.md documentation page and its sidebar entry in sidebars.ts. The AssertionGroup class no longer exists
in the codebase, so this page was orphaned and could confuse users.

Related Issue

Fixes #4966

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update
• Performance improvement
• Refactoring (no functional changes)

Checklist

Required

• I have read the Contributing Guidelines
• If this is a new feature, I started a discussion first and received agreement
• My code follows the project's code style (modern C# syntax, proper naming conventions)
• I have written tests that prove my fix is effective or my feature works

TUnit-Specific Requirements

N/A — docs-only change, no code or test changes.

Testing

• All existing tests pass (dotnet test)
• I have added tests that cover my changes
• I have tested both source-generated and reflection modes (if applicable)

Verified the Docusaurus build (yarn build) succeeds with no errors after the removal.

Additional Notes

Only two files changed:

• Deleted docs/docs/assertions/assertion-groups.md
• Removed 'assertions/assertion-groups' entry from docs/sidebars.ts

[thomhurst]

Thanks!