[MSITE-910] Allow skipping auto-generated breadcrumb item for parent module

**[Konrad Windszus](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=kwin)** opened **[MSITE-910](https://issues.apache.org/jira/browse/MSITE-910?redirect=false)** and commented

Currently it is not possible to **not** render a breadcrumb link for a parent module in a multi module build (https://maven.apache.org/plugins/maven-site-plugin/examples/sitedescriptor.html#Breadcrumbs).

I have the following setup

```
+ grandparent
+ src/site.xml (containing breadcrumbs)
+ parent
+ mymodule
```

In such a setup it is impossible to remove breadcrumb being automatically inserted for `parent`.

Adding another `src/site.xml` to `parent` with an empty `breadcrumbs` section does not help. Adding there at least one `item` overwrites the breadcrumbs from `grandparent` (in case the name does not overlap) but this requires to repeat the breadcrumbs from the `parent`.

Please make the inheritance of breadcrumbs more flexible (similar to https://maven.apache.org/doxia/doxia-sitetools/doxia-decoration-model/decoration.html#menu) to allow to merge breadcrumbs with the parent or to skip automatically adding a breadcrumb.

---

**Affects:** 3.12.1

Start with the Maven Site Plugin breadcrumb inheritance described in MSITE-910 and compare it with the Doxia decoration-model menu inheritance example linked in the issue. The work is done when a child module can either merge inherited breadcrumbs or suppress the automatically inserted parent breadcrumb without repeating the inherited list.

Written by the indexing model from the issue text.