Consolidate Error Sections by Resolution Theme

Replace individual diagnostic H2 sections with thematic H2 sections organized around shared resolution strategies.

Required Input

Collect the destination filename for the consolidated .md file in docs/csharp/language-reference/compiler-messages/.

Key Paths

• Destination: docs/csharp/language-reference/compiler-messages/{destination-filename}
• Reference examples: Other *-errors.md files in docs/csharp/language-reference/compiler-messages/

Phase A: Propose Themes