Streamline validators.md index

Makes it so the index looks more like a cheatsheet, condensing
information instead of making long lists that require lots of
scrolling to explore.

Additionally, the happy path for each validator was also
added, providing a quick reference use for comparison.

The direct markdown links were replaced by titled markdown
references, offering mouse-over tooltips over links that
display the validator one-line description.

Author: alganet