Assembly not always generated for start page when root level is 1

In certain scenarios, the content of the start page is not included in an assembly. This can happen when root level is 1, insert start page is enabled, and the navigation tree does not include the start page.

* xref:use.adoc[]
* xref:configure.adoc[]

The start page is not included in the navigation tree here since the default UI automatically creates a link to the start page. However, we would expect an assembly to still be created for the start page. Otherwise, the content on that page is not available as an export (e.g., PDF).

Assembler should implicitly insert the start page as the first top-level item of the first nav menu in this case, thus creating an assembly for it. It should only do so if the start page is not found in the navigation tree. Assuming the start page is index.adoc, the effective result should be as follows:

* xref:index.adoc[]
* xref:use.adoc[]
* xref:configure.adoc[]

In this case, the start page should act like any other top-level entry. The filename should be based on the slug of the navtitle.

If there is only one nav menu, and the title of nav menu is the start page, the start page should be moved to the first entry in the nav menu.

.xref:index.adoc[<text-or-empty>]
* xref:use.adoc[]
* xref:configure.adoc[]

becomes

* xref:index.adoc[<text-or-empty>]
* xref:use.adoc[]
* xref:configure.adoc[]

If the title of the only nav menu is text, and that text matches the component version title, the title should be ignored (and the start page still inserted into the nav menu, if necessary).