Add more information to VSCode extenstion README

A lot of these are duplicated from the documentation or main README. While it's unfortunate to have duplicated information, the current VSCode page is very barebones and doesn't offer much confidence. 

This updated README offers a few more links and follows a structure similar to the official rust extension and other popular vscode extensions. The additions are, as much as possible specific to the vscode extension and not rust-analyzer as a LSP.

The note about not using the official extension is also right there at the top because that's a common issue people have when trying it out. 

I added the sponsor section since it's common in other extensions README, but I'm not sure if it's necessary

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