NFSD: Add toctree entry for NFSD IO modes docs

Commit fa8d4e6 ("NFSD: add
Documentation/filesystems/nfs/nfsd-io-modes.rst") adds documentation for
NFSD I/O modes, but it forgets to add toctree entry for it. Hence,
Sphinx reports:

Documentation/filesystems/nfs/nfsd-io-modes.rst: WARNING: document isn't included in any toctree [toc.not_included]

Add the entry.

Fixes: fa8d4e6 ("NFSD: add Documentation/filesystems/nfs/nfsd-io-modes.rst")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Closes: https://lore.kernel.org/linux-next/20251202152506.7a2d2d41@canb.auug.org.au/
Signed-off-by: Bagas Sanjaya <bagasdotme@gmail.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Tested-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

Author: bagasme

Documentation/filesystems/nfs/index.rst

@@ -13,5 +13,6 @@ NFS
    rpc-cache
    rpc-server-gss
    nfs41-server
+   nfsd-io-modes
    knfsd-stats
    reexport