Add CompositeFormat class reference to composite formatting docs

[Copilot]

The composite formatting article was missing any mention of System.Text.CompositeFormat (.NET 8+), which pre-parses format strings to eliminate repeated runtime parsing overhead in hot paths.

Changes

• New section — "Improve performance with CompositeFormat" added before See also, covering:
  • How CompositeFormat.Parse() works and why caching the result as a static readonly/Shared ReadOnly field matters
  • Step-by-step usage with String.Format and StringBuilder.AppendFormat
  • Link to the CA1863 analyzer rule that flags optimization opportunities
• Code snippets — C# and VB examples added to the existing snippet files demonstrating CompositeFormat usage
• See also — Added <xref:System.Text.CompositeFormat> and CA1863 rule link

Example (C#)

// Parse once, cache as static readonly
CompositeFormat format = CompositeFormat.Parse("Name = {0}, hours = {1:hh}");

// Pass null for IFormatProvider to use the current culture.
// Use the pre-parsed format to avoid reparsing on every call.
string result = string.Format(null, format, "Fred", DateTime.Now);