Add specific approval PRs to the features page

[alamb]

We now have a https://parquet.apache.org/docs/file-format/versions/ page 🎉

One thing occurred to me while verifying external links...it would be nice for the features to link to their specific PR. The current "Source" column links to all changes between versions, so there's a lot of housekeeping changes to wade through.

Originally posted by @etseidl in #189 (comment)

I think this would basically entail adding a new column to each table with provenance of where it came from