Prior to this commit, the welcome page support implemented in gh-9785
would override existing index views in both annotation and functional
variants.

This comes from the fact that the feature was implemented as a
`RouterFunction` configured in the main `RouterFunctionMapping` bean.
Due to ordering issues between mappings, this would override existing
application mappings in some cases.

This commit ensures that the welcome page `RouterFunction` is
contributed to the context in its own handler mapping, ordered after the
application ones.

Fixes gh-21909