Support role to exclude navigation item from assembly
While Assembler supports profiles for using a different navigation for the assembly, it would be convenient to be able to label a navigation item in the primary navigation to be excluded from the navigation used for the assembly. It's not typical to add navigation entries for the assembly that aren't in the site, but it is typical to want to exclude items.
What I propose is for Assembler to skip an item and its children if it has the role `site-only`.
```
* xref:the-page.adoc[]
* xref:page-to-skip.adoc[role=site-only]
* xref:last-page.adoc[]
```
Recall that conditional directives are only reevaluated for Assembler when the navigation is specified in an assembly profile. That's why it's not enough just to use the conditional directive `ifndef::loader-assembler[]` without using an assembly profile.
issue