NFSD: Add toctree entry for NFSD IO modes docs

Commit fa8d4e6784d1b6 ("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: fa8d4e6784d1b6 ("NFSD: add Documentation/filesystems/nfs/nfsd-io-modes.rst")
Reported-by: Stephen Rothwell <[e-mail]>
Closes: https://lore.kernel.org/linux-next/[e-mail]/
Signed-off-by: Bagas Sanjaya <[e-mail]>
Reviewed-by: Jeff Layton <[e-mail]>
Reviewed-by: Randy Dunlap <[e-mail]>
Tested-by: Randy Dunlap <[e-mail]>
Signed-off-by: Chuck Lever <[e-mail]>

Documentation/filesystems/nfs/index.rst

···
    rpc-cache
    rpc-server-gss
    nfs41-server
+   nfsd-io-modes
    knfsd-stats
    reexport