Update servlet/architecture.adoc to use include-code

[jkuhel]

This PR updates the servlet architecture (servlet/architecture.adoc) docs to use include-code block macro and included some additional unit tests.

Also updated the docs/antora.xml configuration to add include-xml which is configured to docs/src/test/resources. This allowed the include-code macro to include XML-based snippets (e.g. the creation of the nullRequestCache bean).

The only sample that did not use include-code was the following due to the FilterRegistrationBean from the Spring Boot project not being available.

E.g.

@Bean
public FilterRegistrationBean<TenantFilter> tenantFilterRegistration(TenantFilter filter) {
    FilterRegistrationBean<TenantFilter> registration = new FilterRegistrationBean<>(filter);
    registration.setEnabled(false);
    return registration;
}

References #16226

[rwinch]

Thanks for the PR! There were a few changes that I applied:

1. I've updated it to use include-code since include-java is not valid. With that update, I also updated the kotlin code to have the disable tag See 4c9ee66
2. I rebased on main and updated the PR to merge into main. I know the directions said to use 6.5.x (you correctly followed the directions) but the directions were stale so this was a problem on our side (sorry). I've also updated the instructions so that doesn't happen for the next contribution