Document how to trigger the VS Code extension's debugger from launch.json and shell

[acuarica]

Currently, the VS Code extension's debugger can be triggered from three main places:

1. The Truffle: Debug Transaction command,
2. The launch configuration using a launch.json configuration file, or
3. A shell command using the VS Code protocol (implemented in trufflesuite/vscode-ext#231 and trufflesuite/vscode-ext#254)

We need to document both 2. and 3.. In particular, 3. can be used by other tools or scripts that want to interact with the extension's debugger. Moreover, PR trufflesuite/vscode-ext#255 introduces a new flag to disable fetching external sources when debugging a forked network instance trufflesuite/vscode-ext#252.

PR trufflesuite/vscode-ext#255 contains more details and examples on how to trigger the debugger from these places.

cc @kevinbluer