Update regex docs with more info about source gen

[gewarren]

• Inspired by UUF feedback that said that the https://learn.microsoft.com/en-us/dotnet/standard/base-types/compilation-and-reuse-in-regular-expressions didn't include anything about source generation.
• Removed most mentions of CompileToAssembly, since those methods are obsolete since .NET 7.
• Move snippets to live closer to Markdown files and format them.
• Add a source-generator regex sample to the comparison example.
• Format tables.
• Combine thread safety article with best practices.

(Hide whitespace changes.)

---

Internal previews

📄 File	🔗 Preview link
docs/standard/base-types/backtracking-in-regular-expressions.md	Backtracking in regular expressions
docs/standard/base-types/best-practices-regex.md	Best Practices for Regular Expressions in .NET
docs/standard/base-types/compilation-and-reuse-in-regular-expressions.md	Compilation and reuse in regular expressions
docs/standard/base-types/details-of-regular-expression-behavior.md	Regular Expression behavior
docs/standard/base-types/regular-expression-options.md	Regular expression options
docs/standard/base-types/regular-expression-source-generators.md	docs/standard/base-types/regular-expression-source-generators
docs/standard/base-types/the-regular-expression-object-model.md	The regular expression object model