docs(csharp): add mTLS examples to TLS page

[Aryex]

Summary

Adds C# code examples for all four mTLS subsections on the Configure TLS page, replacing the "Coming soon" placeholders.

⚠️ Attention: This PR was generated automatically. Verify that all relevant pages have been updated.

Source Commits

• cb6f3d5 — feat(config): Add mutual TLS (mTLS) client certificate support (#500)

Changes

• Replaced C# "Coming soon" placeholders in the mTLS section with complete code examples:
  • In-memory PEM (static) — WithClientCertificate(byte[], byte[])
  • Path-based with automatic reload — WithClientCertificate(string, string)
  • Path-based with custom reload interval — WithClientCertificate(string, string, TimeSpan)
  • Loading PEM material from files — File.ReadAllBytes + WithClientCertificate(byte[], byte[])
• Added C#-specific bullet to the Common Pitfalls section (path vs bytes exclusivity, TimeSpan rounding, ArgumentOutOfRangeException)

Context

C# mTLS support (both byte-based and path-based with automatic reload at configurable intervals) shipped in valkey-glide-csharp #500. This extends the same feature previously landed for Java, Go, Python, and Node.js. The existing doc PRs #271 (Java), #304 (Python), and #311 (Node.js) cover other languages. This PR adds the C# examples independently.

cc @currantw

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This PR was generated by the automated documentation pipeline.