docs: use reference-style markdown links

[gma]

Problem

I cloned the repository and cracked open the README in my editor, and found it hard to read the features list with all the embedded links.

See https://github.github.com/gfm/#link-reference-definitions for docs on reference links.

Changes

• Converted Markdown links in the README's list of features to Markdown's reference-style

How did you test this code?

N/A

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