fix(web): bundle docs sources at build time so the worker has no fs

The worker deploy from #3859 returned HTTP 500 on /docs with
`Error: [unenv] fs.readFile is not implemented yet!` (captured via
`wrangler tail`). `loadDocSource` was calling `node:fs/promises`
`readFile` inside an RSC; even though the page is generated as SSG
(`●`), Cloudflare Workers' unenv shim does not implement filesystem
reads, so any code path that reaches the worker (cache miss, prerender
fallback) fails.

Move the read to a prebuild step that emits a TypeScript module:

- `web/scripts/generate-docs-content.mjs` reads each section's source
  from `<repo-root>/docs/` and writes
  `web/lib/docs-content.generated.ts` containing
  `export const DOC_SOURCES: Record<string, string>`.
- `web/scripts/prepare-build.mjs` invokes the generator after the
  cache prune, so every `bun run build` and `bunx opennextjs-cloudflare
  build` regenerates the constant module from the live `docs/`.
- `web/lib/docs-source.ts` now reads `DOC_SOURCES[file]` synchronously
  with no Node I/O.
- `web/app/[locale]/docs/page.tsx` drops the `Promise.all` since reads
  are synchronous.
- `web/.gitignore` excludes the generated file (kept generated, not
  source-of-truth).

Effect: the bundle ships every doc as a string literal. The worker has
no `fs.readFile` call to fail. `docs/` remains the only place an
editor needs to touch.

web/lib/docs-source.ts

@@ -1,9 +1,9 @@
-import { readFile } from "node:fs/promises"
-import path from "node:path"
+import { DOC_SOURCES } from "./docs-content.generated"
 
-const DOCS_ROOT = path.resolve(process.cwd(), "..", "docs")
-
-export async function loadDocSource(file: string): Promise<string> {
-  const fullPath = path.join(DOCS_ROOT, file)
-  return readFile(fullPath, "utf8")
+export function loadDocSource(file: string): string {
+  const source = DOC_SOURCES[file]
+  if (source === undefined) {
+    throw new Error(`Unknown doc file: ${file}`)
+  }
+  return source
 }