LU-12275 sec: documentation for client-side encryption

LU-12275 sec: documentation for client-side encryption

Add several documents about client-side encryption under
Documentation/client_side_encryption:
- threat_model.txt is the description of the threat model for Lustre
  client-side encryption;
- key_hierarchy.txt is the description of the key hierarchy for Lustre
  client-side encryption;
- modes_usage.txt is the description of the encryption modes and usage
  for Lustre client-side encryption;
- access_semantics.txt is the description of the access semantics for
  Lustre client-side encryption.

As we rely on kernel's fscrypt library for this feature, fscrypt's
concepts are largely valid. These documents are inspired by fscrypt
documentation in the Linux kernel tree, see
Documentation/filesystems/fscrypt.rst

Test-Parameters: trivial
Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: I6c9d42e572111ed2a3388e4f58b2560f365a5853
Reviewed-on: https://review.whamcloud.com/38759
Reviewed-by: John L. Hammond <jhammond@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>