chore: use "Sentence case" consistently in the navigation tree

[roman-kashitsyn]

Currently, different nodes of the navigation tree use inconsistent title capitalization rules.

This change makes all the titles use "Sentence case" (as opposed to "Title Case") because this style is used in most of the documents (e.g., in the public interface spec, Motoko guides, Rosetta docs), so switching to "Sentence case"is less work than the other way around.

Most IC wiki articles also use the "Sentence case" for headers.

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