Group by maturity in the default table

[jsignell]

Closes #43

I ran it locally to make sure that it works properly. Seems like it's all good. Within each maturity level the extensions are alphabetical

[m-mohr]

I personally always look for the extensions alphabetically, taking it away completely feels like a step too far.

Also, unless we have a solid mechanism for updating the maturity, this might put a bit too much focus on the premature(?) maturity levels yet.

[jsignell]

I recognize that this is a change but it's not clear to me that you actually lose anything. Alphabetical order doesn't contain any meaningful information. If you want to find the proj extension wouldn't you just ctrl + f for it and if you are just browsing why would you care that you see the extensions that start with "A" first?

[m-mohr]

Sure, you can CTRL+F, but for whetever reason I always just scroll through. Similarly, when I'm in calls with people and we discuss specific extensions, I see most people just go to the table and scroll through to the extension.

I'm not against the regrouping in general, but I don't see a reason why we not just provide both.

But my main point is: Let's first establish a way to actually evaluate and update the maturity and execute it, before putting too much emphasis on it. Currently, the the maturity levels are a couple of years old and it misleads, e.g. many commonly used extensions are still in proposal (product, auth, ...)