chore: clean up

[innocenzi]

It's been a while since our last clean up, it was long overdue! This pull request slightly updates the contribution guidelines, the template, and most importantly, the list.

• A lot of links have been removed. Most of them were out-of-date (still on Tailwind CSS v3, some even on v2 or v1!), a few of them were hijacked or down, and some were simply no longer useful.
• I removed the "starters and theme" and "learning" categories. Both of them had very outdated links, often being more than 6 years old. There were barely enough resources there to justify those categories.
• I don't think it makes sense anymore to have the plugin category first. It was useful until Tailwind CSS v4, but this version is so much more flexible that the needs for plugins basically vanished. I put it last for this reason.

[joshmanders]

LGTM!