#193 docs: add project and architecture guides

Summary

• Add project overview and architecture guides to the documentation site.
• Link the new guides from the site navigation, overview page, and repository README.
• Add a docs Changeset.

Verification

• bun run --filter @mynameistito/opencode-plugins-docs test
• bun run --filter @mynameistito/opencode-plugins-docs build
• git diff --check

The docs test/build passed. The test emitted a warning that GITHUB_TOKEN is not set for the local GitHub releases content source.

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Summary by cubic

Adds project overview and architecture guides to the documentation site and fixes the link-check setup so the new docs resolve correctly in CI.

• New /project and /architecture pages cover repository layout, packages, plugin runtime flows, and deployment boundaries.
• Links the new guides from the site navigation, overview page, and repository README.
• Updates the link-check workflow to resolve docs-relative links and adds the two npm package pages to .lycheeignore.
• Includes a docs Changeset for the site content.