Add language reference material for unsafe updates for memory safety

[BillWagner]

Fixes #54011

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Internal previews

📄 File	🔗 Preview link
docs/csharp/language-reference/keywords/unsafe.md	unsafe (C# Reference)
docs/csharp/language-reference/operators/pointer-related-operators.md	Pointer related operators - take the address of variables, dereference storage locations, and access memory locations
docs/csharp/language-reference/operators/sizeof.md	"sizeof operator - determine the storage needs for a type"
docs/csharp/language-reference/operators/stackalloc.md	stackalloc expression (C# reference)
docs/csharp/language-reference/statements/fixed.md	docs/csharp/language-reference/statements/fixed
docs/csharp/language-reference/unsafe-code.md	Unsafe code, pointer types, and function pointers
docs/csharp/misc/sorry-we-don-t-have-specifics-on-this-csharp-error.md	Coming in C# 15
docs/csharp/specification/toc.yml	docs/csharp/specification/toc
docs/csharp/whats-new/csharp-15.md	What's new in C# 15

[Copilot]

Pull request overview

This PR adds C# 15 (preview) documentation for the evolving memory safety model around unsafe, and wires the related feature-spec proposal into the documentation set.

Changes:

• Adds a new “Memory safety” section to the C# 15 “What’s new” page.
• Adds a “Memory safety (preview)” section to the unsafe code reference and updates related keyword/operator/statement reference pages.
• Adds a new C# snippet project used by the memory safety docs, and updates docfx/toc wiring for the proposal.

Reviewed changes

Copilot reviewed 13 out of 13 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
docs/csharp/whats-new/csharp-15.md	Adds a C# 15 “Memory safety” feature overview and link to the spec/proposal.
docs/csharp/specification/toc.yml	Adds a “Unsafe code > Memory safety” entry under feature specifications.
docs/csharp/misc/sorry-we-don-t-have-specifics-on-this-csharp-error.md	Renames an internal keyword grouping comment from “unsafe evolution” to “memory safety”.
docs/csharp/language-reference/unsafe-code.md	Adds “Memory safety (preview)” reference content and snippet inclusions.
docs/csharp/language-reference/statements/fixed.md	Adds a note about the preview relaxations for fixed.
docs/csharp/language-reference/snippets/memory-safety/Relaxations.cs	New snippet source demonstrating memory safety relaxations.
docs/csharp/language-reference/snippets/memory-safety/Program.cs	New entry point to build/verify the snippet project.
docs/csharp/language-reference/snippets/memory-safety/memory-safety.csproj	New snippet project targeting net11.0 with LangVersion=preview.
docs/csharp/language-reference/operators/stackalloc.md	Adds a note about preview relaxations for stackalloc→pointer conversion.
docs/csharp/language-reference/operators/sizeof.md	Adds a note about preview relaxations for sizeof on unmanaged types.
docs/csharp/language-reference/operators/pointer-related-operators.md	Adds a note about preview relaxations for the address-of operator.
docs/csharp/language-reference/keywords/unsafe.md	Adds a summary note about the memory safety preview changes.
docfx.json	Adds the unsafe-evolution proposal to proposal content/title/description mappings.