Add introductory guide to .NET source-build

[mthalman]

Summary

.NET engineers need a consolidated introduction to the source-build philosophy and the downstream constraints it protects. This adds a teaching-oriented guide that explains why source-build exists before introducing its implementation details.

The guide covers:

• source-build users, goals, and downstream guarantees;
• the VMR build lifecycle and dependency provenance;
• repository ordering and package-version lifting;
• prebuilt detection, poisoning, and binary policy;
• sequential and two-stage bootstrap workflows; and
• source-build validation and CI dimensions.

The documentation index now links to the new guide.

Validation

• Verified local Markdown files and heading anchors resolve.
• Verified reference-style links are defined and used.
• Checked the documentation diff for formatting errors.

[ellahathaway]

It might be helpful to reference this documentation in https://github.com/dotnet/dotnet/blob/main/.github/skills/source-build-investigation/SKILL.md

[mthalman]

It might be helpful to reference this documentation in https://github.com/dotnet/dotnet/blob/main/.github/skills/source-build-investigation/SKILL.md

dotnet/dotnet#7991