fs-verity: add a documentation file

Add a documentation file for fs-verity, covering:

- Introduction
- Use cases
- User API
    - FS_IOC_ENABLE_VERITY
    - FS_IOC_MEASURE_VERITY
    - FS_IOC_GETFLAGS
- Accessing verity files
- File measurement computation
    - Merkle tree
    - fs-verity descriptor
- Built-in signature verification
- Filesystem support
    - ext4
    - f2fs
- Implementation details
    - Verifying data
        - Pagecache
        - Block device based filesystems
- Userspace utility
- Tests
- FAQ
Reviewed-by: NTheodore Ts'o <tytso@mit.edu>
Reviewed-by: NJaegeuk Kim <jaegeuk@kernel.org>
Signed-off-by: NEric Biggers <ebiggers@google.com>

Documentation/filesystems/index.rst

@@ -32,3 +32,4 @@ filesystem implementations.
 
    journalling
    fscrypt
+   fsverity