Add documentation for CA1514

[mpidash]

See issue dotnet/runtime#68946 and PR dotnet/roslyn-analyzers#6814.

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Internal previews

📄 File	🔗 Preview link
docs/fundamentals/code-analysis/quality-rules/ca1514.md	CA1514: Avoid redundant length argument
docs/fundamentals/code-analysis/quality-rules/index.md	Code quality rules overview
docs/fundamentals/code-analysis/quality-rules/maintainability-warnings.md	Maintainability rules

[gewarren]

Thanks for adding the docs. I left a few suggestions (our grammar checker doesn't like sentences that start with -ing words).