README.md: Remove sample output; link to HTML

Instead of duplicating information here, bring the HTML link above the fold
and refer to the generated page for more information about the tool.
encodings.py: HTML: make <h3> headers clickable

Headers link to themselves for easy bookmarking.

Change the anchor name to the more-elegant #FF instead of #0xFF but
keep the old syntax available as a synonym for legacy links to this
page.