DOC Add documentation entry about file system access (#2420)

Co-authored-by: Roman Yurchak <rth.yurchak@gmail.com>
Co-authored-by: Hood Chatham <roberthoodchatham@gmail.com>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

docs/usage/file-system.md

@@ -0,0 +1,49 @@
+(file-system)=
+
+# Dealing with the file system
+
+Pyodide includes a file system provided by Emscripten.
+In JavaScript, the Pyodide file system can be accessed through {any}`pyodide.FS` which re-exports the [Emscripten File System API](https://emscripten.org/docs/api_reference/Filesystem-API.html#filesystem-api)
+
+**Example: Reading from the file system**
+
+```js
+pyodide.runPython(`
+  with open("/hello.txt", "w") as fh:
+      fh.write("hello world!")
+`);
+
+let file = pyodide.FS.readFile("/hello.txt", { encoding: "utf8" });
+console.log(file); // ==> "hello world!"
+```
+
+**Example: Writing to the file system**
+
+```js
+let data = "hello world!";
+pyodide.FS.writeFile("/hello.txt", data, { encoding: "utf8" });
+pyodide.runPython(`
+  with open("/hello.txt", "r") as fh:
+        data = fh.read()
+  print(data)
+`);
+```
+
+## Mounting a file system
+
+The default file system used in Pyodide is [MEMFS](https://emscripten.org/docs/api_reference/Filesystem-API.html#memfs),
+which is a virtual in-memory file system. The data stored in MEMFS will be lost when the page is reloaded.
+
+If you wish for files to persist, you can mount other file systems.
+Other file systems provided by Emscripten are `IDBFS`, `NODEFS`, `PROXYFS`, `WORKERFS`.
+Note that some filesystems can only be used in specific runtime environments.
+See [Emscripten File System API](https://emscripten.org/docs/api_reference/Filesystem-API.html#filesystem-api) for more details.
+For instance, to store data persistently between page reloads, one could mount
+a folder with the
+[IDBFS file system](https://emscripten.org/docs/api_reference/Filesystem-API.html#filesystem-api-idbfs)
+
+```js
+let mountDir = "/mnt";
+pyodide.FS.mkdir(mountDir);
+pyodide.FS.mount(pyodide.FS.filesystems.IDBFS, { root: "." }, mountDir);
+```

docs/usage/index.md

@@ -120,4 +120,5 @@ For this same reason, installing Pyodide packages from the CDN is explicitly not
 
    webworker.md
    loading-custom-python-code.md
+   file-system.md
 ```