fix site descriptor (revert partially PR #161)

[hboutemy]

when upgrading to this parent POM, child projects must do 2 things:

1. explicitely declare a url in their pom.xml (they currently usually don't have it, but just inherited default value that is now wrong)
2. override the whole breadcrumbs in their site.xml

= for example https://github.com/codehaus-plexus/plexus-xml/pull/18/files

understanding the why:

1. before PR Refresh Maven site #161, url was expected to be automagically inherited to child from parent value and child artifactId; This was done at the cost of parent POM inconsistency between its url and the content of site.xml, that lead to complex understanding of urls relativisation. Let's keep it simple and just declare <url> in each child project
2. in child projects, by default, breadcrumbs from parent will be used and additionam breadcrumb will be used: this leads to a breadcrumb to parent POM project = not something we expect. Redeclaring full breadcrumbs from the start will override content from parent

a little bit of copy paste will make things much simpler than the initial intended magic that was causing headache

[slawekjaranowski]

Do we really need a link to parent in breadcrumbs?
We have a link to parent in menu items ...
@hboutemy @michael-o