Make PageIndex::new public, add doc example of building a synthetic PageIndex

[alamb]

Which issue does this PR close?

• Closes parquet: PageIndex cannot be constructed outside the crate #10824.

Rationale for this change

#10719 introduced the PageIndex struct but its constructor is pub(crate), so external code can no longer assemble ColumnIndexMetaData / OffsetIndexMetaData values (whose builders are still public) into a ParquetMetaData. DataFusion relies on this to construct synthetic metadata in tests that exercise page-level statistics handling (see apache/datafusion#24366).

What changes are included in this PR?

• Make PageIndex::new pub and document its arguments
• Add a doc example showing how to construct a synthetic PageIndex and attach it to a ParquetMetaData via ParquetMetaDataBuilder::set_page_index, modeled on the DataFusion test code that needs this API

Are these changes tested?

Yes, by the new doc example.

Are there any user-facing changes?

PageIndex::new is now part of the public API. No breaking changes.

[etseidl]

Perhaps I could interest you in a builder 😅 #10784

[alamb]

Perhaps I could interest you in a builder 😅 #10784

Yes! I was just reviewing this PR and the signature for new was ugly and I had just decided I was going to have claude whip up a builder. Even better that you have one (you are already ahead!). Let's go with that

[etseidl]

I want am going to steal your example and merge it into the builder PR