doc: Add documentation for file system subsystem

Add documentation for file system subsystem.

Signed-off-by: Ramakrishna Pallala <[email protected]>

Author: Ramakrishna Pallala

doc/reference/file_system/index.rst

@@ -3,13 +3,54 @@
 File Systems
 ############
 
+Zephyr RTOS Virtual Filesystem Switch (VFS) allows applications to mount multiple
+file systems at different mount points (e.g., ``/fatfs`` and ``/nffs``). The
+mount point data structure contains all the necessary information required
+to instantiate, mount, and operate on a file system. The File system Switch
+decouples the applications from directly accessing an individual file system's
+specific API or internal functions by introducing file system registration
+mechanisms.
 
+In Zephyr, any file system implementation or library can be plugged into or
+pulled out through a file system registration API.
 
-.. comment
-   not documenting
-   .. doxygengroup:: file_system
-   .. doxygengroup:: file_system_storage
+.. code-block:: c
 
+        int fs_register(enum fs_type type, struct fs_file_system_t *fs);
+
+        int fs_unregister(enum fs_type type, struct fs_file_system_t *fs);
+
+Zephyr RTOS supports multiple instances of a file system by making use of
+the mount point as the disk volume name, which is used by the file system library
+while formatting or mounting a disk.
+
+A file system is declared as:
+
+.. code-block:: c
+
+	static struct fs_mount_t mp = {
+	.type = FS_FATFS,
+	.mnt_point = FATFS_MNTP,
+	.fs_data = &fat_fs,
+	};
+
+where
+
+- ``FS_FATFS`` is the file system type like FATFS or NFFS.
+- ``FATFS_MNTP`` is the mount point where the file system will be mounted.
+- ``fat_fs`` is the file system data which will be used by fs_mount() API.
+
+Known Limitations
+*****************
+
+NFFS supports only one instance of file system due to the library's internal
+implementation limitation.
+
+
+Sample
+******
+
+A sample of how the file system can be used is supplied in ``samples/subsys/fs``.
 
 API Reference
 *************