Add warning regarding title field in index

[fabienheureux]

Add W005 warning when SearchField("title") is missing

Summary

I noticed (accidentally) that title field name is special in Django ModelSearch: a SearchField named "title" populates a dedicated title index column used for ranking. If no such field is declared, the title column is silently left empty, degrading search quality with no indication to the developer.

This PR adds a system check warning (modelsearch.W005) that fires when a model defines at least one SearchField but none of them is named "title".

I think this is a wagtail's legacy where title is always defined

Example warning output

myapp.Article: (modelsearch.W005) Article.search_fields does not contain a SearchField named 'title'. The title index column will be empty.
    HINT: Add index.SearchField('title') to Article.search_fields to populate the title index column, which is used for ranking search results.