[MRG] Improve documentation for CHI and DBI clustering metrics (#12764)
[MRG] Improve documentation for CHI and DBI clustering metrics (#12764)

* Improve / reorganize and clarify docs for Calinski-harabasz Index
 - Corrects grammer and adds punctuation
 - Adds "Mathematical formulation" subsection
 - Improves math description and symbols

* Correct reference for Calinski-Harabasz Index

* Create math section for Davies-Bouldin Index information

* Remove unsourced claim in Davies-Bouldin score docs

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