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.

Author: Chris Heller <hellertime@gmail.com>
Author: Chris Heller <cheller@akamai.com>

Closes #1 from hellertime/feature/improve-encoding-doc and squashes the following commits:

c2e5f54 [Chris Heller] Link the delta encoding section
c1e85d2 [Chris Heller] Add anchor links