Update docs landing page

[MaryaBelanger]

I think the docs "landing page" https://dart.dev/guides should be updated to better represent the contents of the site as laid out in the side nav. The "futures..." box doesn't really fit here either.

So the boxes should be:

• Tutorials and codelabs
• Language
• Effective Dart
• Core libraries
• Packages
• Development
• Tools & techniques
• Resources

And each block should accurately summarize the contents.

[parlough]

Great point! I guess I don't consider this page that often.

I wish we had a 4-5 minute "Introduction to Modern Dart" video or something to stick at the bottom of the page too, but I couldn't find anything that fits well yet.

[parlough]

Also @MaryaBelanger I thought this page could be a helpful source of inspiration - https://go.dev/doc

Depending on what you're thinking and would like to do, I can help you figure out how to pull page descriptions from the SSG (Jekyll/11ty/etc).