Improve Extension Documentation

[jpogran]

This PR updates the README with more user focused documentation about extension usage, features, troubleshooting info, an improved quick start, and more detailed workflow scenarios. The aim is to be as comprehensive and helpful as possible without going into dedicated website territory.

To do this required significantly changing the order and content of the sections. This allows us to have linkable headers that we can provide in tickets and when interacting with our community.

It also adds example screenshots of features in use. These are purposely ignored by the packaging command to save space in the VSIX. They are referenced using Markdown links that will be automatically translated by vsce when packaging, so will be available to internet connected clients when viewed inside the Extension pane. If there is no internet, they won't be resolvable, but that is a small price to pay for a small install size.

This also includes corrected and/or updated information that replaces the following Wiki pages:

• https://github.com/hashicorp/vscode-terraform/wiki/Install-a-Pre-transfer-Version
• https://github.com/hashicorp/vscode-terraform/wiki/Manually-Setting-the-Terraform-Executable-Path

[dbanck]

Amazing work! This is super helpful!

[radeksimko]

FWIW the language server should be able to pick up new providers automatically whenever anything changes in .terraform, so init should actually be sufficient here - there should be no need to restart VSCode.

[jpogran]

This was hard to figure out. The original documentation was from when we had to run init outside the editor and I was trying to reconcile that with current experience. I'll edit this.

[jpogran]

Updated

[dbanck]

Great README update!

I've commented on some minor findings. Also, docs/syntax.png is unused and could be removed?

[github-actions]

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