Write documentation for Security's HashProcessor

[talevy]

#31087 added the implementation for
the hash processor, but it is lacking documentation.

A proper place for security's ingest processor documentation needs to be found,
and the hash processor's reference docs and examples needs to be added.

[elasticmachine]

Pinging @elastic/es-core-infra

[talevy]

opened #32112 to close this

[talevy]

Closing due to ongoing work here: #34085