[C++][Parquet] PageIndexReader is not externally documented

[alexeyroytman]

The "PageIndexReader" seems not to be externally documented. The search:
https://arrow.apache.org/docs/search.html?q=PageIndexReader
returns just 2 mentions, without any documentation of class' methods.
Is this supposed to be this way (maybe, because of some work-in-progress) ?

[kou]

I think that we need to add an API reference document for cpp/src/parquet/. We have API reference document only for cpp/src/parquet/arrow/ for now.

I'll create an issue from this.

[kou]

Let's use #49983 for further discussion.