chore: add onflow-docs submodule, build scripts, remove Docusaurus artifacts

- .gitmodules + external/onflow-docs: pin onflow/docs submodule for federated
  Orama search (cross-site hits link to developers.flow.com)
- scripts/generate-sitemap.ts: post-build sitemap generator
- scripts/check-links.mjs: offline link-checker for MDX cross-references
- scripts/update-ai-files.ts: utility to sync AI-facing content files
- Remove docusaurus.config.js and package-lock.json (superseded by
  source.config.ts and bun.lock respectively)

Author: Aliserag

.gitmodules

@@ -0,0 +1,3 @@
+[submodule "external/onflow-docs"]
+	path = external/onflow-docs
+	url = https://github.com/onflow/docs.git