docs: add README comparison with similar tools and usage guidance

[39ff]

Motivation

• Clarify where this project sits in the observability ecosystem by comparing it to APMs, DB log analyzers, and low-level mysqlnd hooking approaches.
• Help users decide when to adopt this extension by providing concrete usage scenarios and trade-offs.

Description

• Added a new Comparison with Similar Tools section to README.md that compares PHP APM/profilers, DB log analysis tools, and mysqlnd-hooking approaches in a compact table.
• Added a Why this project is different subsection that highlights mysqlnd-layer SQL capture, optional PHP backtraces, tag stack APIs, job-oriented CLI operations, and local-first usage.
• Added a When You Should Use This Project section listing five concrete conditions favoring this tool and a Prefer alternatives when... list that clarifies common trade-offs.
• Only README.md was modified to include the new documentation content.

Testing

• Ran formatting and whitespace checks with git diff --check and they passed.
• Inspected the staged changes with git diff -- README.md and reviewed the updated content, which matched the intended additions.
• Verified the final file contents with nl -ba README.md | sed -n '20,120p' to confirm the new sections are present and correctly ordered.

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