Reorganize the extensions table

[matthewhanson]

The initial list a visitor sees at the https://stac-extensions.github.io/ page is rather daunting, at 57 extensions all in a variety of states.

Later in the page is a useful listing of the extensions by maturity level, but it's just a list without the descriptive info, meaning you have to cross reference back to the alphabetical listing to learn about what they do.

I think it would be better to have the initial listing be the full tables but grouped by maturity level so that a visitor can digest what's available by the most common and mature extensions first. An alphabetical listing could be added at the end as a simple list.