Fix README and docs pages that no longer match the code

Bring the READMEs and the docs pages (en and ja) in line with the
current header:

- Fix the client sample that does not compile, the Transfer-Encoding
  statement for default headers, the verifier callback comment, the
  thread pool spawn rule, the sanitize_filename description, and the
  inverted Accept-Encoding sentence.
- Describe recent behaviour: TLS errors behind Read/Write failures,
  Host validation, Connection: close for a body without length,
  content_reader rules, refused CONNECT, WebSocketClient proxies,
  CPPHTTPLIB_INTERIM_RESPONSE_MAX_COUNT, the SSE event size limit, and
  the payload limit of the Stream API.
- Complete the API references of WebSocket, WebSocketClient and the
  stream:: helpers, and the SSE reconnect rules.
- Fix cookbook pages: keychain loading on every backend, received Close
  status, a multipart sample that trusted the client's file name, an
  echo sample that sent binary as text, SNI, pre-request handler scope,
  stop() with WebSocket handlers, and the thread pool notes.
- Restore the mutex in the LLM app's Chapter 5 code.

docs-src/pages/en/cookbook/s04-static-files.md

@@ -10,12 +10,19 @@ To serve static files like HTML, CSS, and images, use `set_mount_point()`. Just
 
 ```cpp
 httplib::Server svr;
-svr.set_mount_point("/", "./public");
+if (!svr.set_mount_point("/", "./public")) {
+  // ./public doesn't exist
+  return 1;
+}
 svr.listen("0.0.0.0", 8080);
 ```
 
+`set_mount_point()` returns `false` when the directory doesn't exist, so check the return value.
+
 `./public/index.html` is now reachable at `http://localhost:8080/index.html`, and `./public/css/style.css` at `http://localhost:8080/css/style.css`. The directory layout maps directly to URLs.
 
+A request for a directory serves its `index.html`, and one without the trailing slash (`/docs`) is redirected (301) to `/docs/`. A symlink that resolves outside the mounted directory gets a 403.
+
 ## Multiple mount points
 
 You can register more than one mount point.