docs: add project mdBook

[philgweber]

This PR:

• Establishes project-owned documentation in line with governance RFC 0044.
• Migrates the Secure EC Services page into mdBook with Mermaid diagrams.
• Adds PR validation and main-branch GitHub Pages deployment.
• Adds a README discovery link.
• A coordinated removal PR will follow.

Closes OpenDevicePartnership/pfd-devops#218

[Copilot]

Pull request overview

This PR introduces project-owned documentation using mdBook (with Mermaid diagrams) and adds GitHub Actions automation to build and deploy the docs to GitHub Pages, with a README link for discovery.

Changes:

• Add an mdBook documentation site scaffold (book config, SUMMARY, and initial “Secure EC Services” page with a Mermaid diagram).
• Add a GitHub Actions workflow to build mdBook on PRs and deploy to GitHub Pages on pushes to main.
• Add a README “Documentation” section and ignore the generated docs/book/ output.

Reviewed changes

Copilot reviewed 6 out of 8 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
README.md	Adds a discovery link pointing readers to the mdBook source.
docs/src/SUMMARY.md	Defines the mdBook table of contents.
docs/src/secure_ec_services.md	Adds initial documentation page with Mermaid diagram content.
docs/mermaid-init.js	Initializes Mermaid and handles theme switching behavior.
docs/book.toml	Configures mdBook build/output and Mermaid preprocessor integration.
.gitignore	Ignores generated mdBook output directory (docs/book/).
.github/workflows/docs.yml	Builds mdBook on PRs and deploys to GitHub Pages on main.

Suppressed comments (1)

docs/src/secure_ec_services.md:19

• The diagram label uses “ARM/FFA” while the page text uses “FF-A”. Consider using “FF-A” here as well for consistency.

    A1 -->|"Secure Path (ARM/FFA)"| B1

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