* Add relevant documentation for FIPS 140-2 compliance.
* Introduce `fips_mode` setting.
* Discuss necessary configuration for FIPS 140-2
* Discuss introduced limitations by FIPS 140-2
* [DOCS] Add configurable password hashing docs
Adds documentation about the newly introduced configuration option
for setting the password hashing algorithm to be used for the users
cache and for storing credentials for the native and file realm.
This commit adds documentation for configuring Kerberos realm.
Configuring Kerberos realm documentation highlights important
terminology and requirements before creating Kerberos realm.
Most of the documentation is centered around configuration from
Elasticsearch rather than go deep into Kerberos implementation.
Kerberos realm settings are mentioned in the security settings
for Kerberos realm.
Ioannis Kakavas
Yogesh Gaikwad
Lisa Cawley