Add a warning about cases of two words terms appear in Index few times because of case sensivity

[ajluk]

During checking the differences between easybaked and kitchen baked Sociology 3e I have found one difference in Index. There is a term racial profiling which is also present in the book written using uppercase: Racial Profiling and during baking in Kitchen these two terms are added to Index separately instead collapse to one term:

Here is how the term is used in baked book:

The goal is to add a warning in the code where these differences can be detected so they can be used by developers to alert CM about such cases in appearing in the books.
Similar warning has been created to alert devs about missing lang attribute here: https://github.com/openstax/kitchen/blob/a642d6d510d12ae38eaae1e9b0d949527e8715a2/lib/kitchen/document.rb#L169

[lwtchu]

Changed naming conventions and this is no longer relevant

[ajluk]

@lwtchu I'm reopening because I have misunderstood you. I understood that you were asking about old cnx-recipes issue, not kitche one.