Update reactive/authentication/logout.adoc to use include-code

[lu1tr0n]

This PR updates docs/modules/ROOT/pages/reactive/authentication/logout.adoc to use the include-code macro for its Java and Kotlin samples.

The custom logout handler sample is now sourced from CustomLogoutHandlerConfiguration (Java and Kotlin), tagged as customLogoutHandler, and pulled into the rendered docs via include-code::./CustomLogoutHandlerConfiguration[tag=customLogoutHandler,indent=0].

Verification:

• ./gradlew :spring-security-docs:compileTestJava :spring-security-docs:compileTestKotlin passes.
• ./gradlew :spring-security-docs:antora renders the page; the resulting reactive/authentication/logout.html shows the Java/Kotlin tabs with the expected snippets and no unprocessed include-code macros.

References #16226

[lu1tr0n]

Hi @rwinch, hope you're doing well — just a gentle nudge on this one whenever you have a moment. It's a small include-code conversion for reactive/authentication/logout.adoc, following the same pattern as #18856 (websocket) and #18536 (servlet/architecture).

The label is still status: waiting-for-triage, so if it would help I'm happy to rebase on current main, adjust the snippet tags, or split it differently — whatever makes it easiest to review. Also happy to defer to @jzheaux if he's closer to this area of the docs.

Thanks for all the work you do on Spring Security!