Add Kotlin samples to the reference documentation

[eleftherias]

Summary

We have many Java and XML samples in the reference documentation.
We should expand these to include Kotlin samples.

For an example of how to add Kotlin samples, take a look at 603eb1e.

Contributors taking on this task are encouraged to break it up into multiple PRs that add samples to one file at a time.

Additionally, contributors should comment on this issue to state which files they are going to start adding samples to. This will avoid duplicating efforts.

[eleftherias]

I'm starting password-storage.adoc

[eleftherias]

I'm starting filters.adoc

[eleftherias]

I'm starting delegating-filter-proxy.adoc and security-context-holder.adoc

[eleftherias]

I'm starting crypto/index.adoc and preauth.adoc

[eleftherias]

I'm starting reactive/webflux.adoc