Landing page for VSCode extension on install

[Veykril]

We should open an information tab when a user installs the VSCode extension, showing a bunch of useful features and configurations the user might be interested in. This should help with discoverability for features that we do not want to enable by default.

Came up in https://rust-lang.zulipchat.com/#narrow/stream/122651-general/topic/RA.20appreciation.20thread/near/302443767

[lnicola]

We should probably make it a page on rust-analyzer.github.io and open that in a tab. On the other hand, there was a request to show the release notes, which is more awkward.