Documentation for the Swift extension for VS Code

[matthewbastien]

Currently, there are two locations for documentation about how to use the Swift extension for Visual Studio Code:

1. A page on https://www.swift.org/tools/ which outlines how to install the extension and has sections for the most commonly used features
2. A folder within this repository that goes over a few features and contains documentation for contributors. Right now this is very bare-bones and does not cover a whole lot of features.

Microsoft also has a documentation page giving overviews of features for various languages that can be contributed to by the community in the VS Code documentation repository. This could be a great place to contribute to as well.

Investigate options for making contributions to these various sources and try to make documentation easier to find for users of the Swift extension for VS Code. It should also be as simple as possible to contribute to these various sources for contributors to the extension.

[matthewbastien]

Putting in my two cents: I've seen other Swift repositories using swift-docc to generate documentation from markdown. This is then compiled into a single page web application that can be published as a GitHub webpage which could be a valid replacement for the simple markdown content that exists for the extension right now.