Add anchor links

I'm starting work on a Haskell implementation of the Parquet format and wanted to get up to speed on the various encodings used.

I find that the current Encodings.md document is quite confusing.

This is a first pass at some clean-up.

There were multiple references in the text that made no sense, as they were references to sections which appear to have been re-ordered in the document.

I've rewritten those references as anchor links within the document to avoid future issues.