Add public FIPS documentation

[naemono]

Proposal

We recently merged FIPS changes in which we also release a FIPS enabled version of the ECK Operator during the standard release process. We need to add some public documentation for this feature.

[pebrc]

I think we should also fix the release notes for 2.6 to mention this new feature.

[perezjasonr]

Hello, sorry I know this is closed, but this is just the operator right? the fips operator image doesn't use different fips specific conf (like xpack.security.fips_mode.enabled) and fips specific images
for the actual es images, kibana, rest of the stack, (like is there an elasticsearch-fips image?) etc right?
is there any docs for that? or is it up to the user to build custom images and/or translate the stuff from

https://www.elastic.co/guide/en/elasticsearch/reference/current/fips-140-compliance.html

to be done via eck-operator way, which looking at it, are all those things possible through the operator?