Docs: Add info on installing VSCode desktop extensions

[stirby]

A customer recently noted that we have no guides on installing VSCode extensions for newly created workspaces. This is a common use case and relatively easy to automate with the VSCode CLI.

# install the VSCode CLI
curl -Ls https://code.visualstudio.com/sha/download?build=stable&os=cli-alpine-x64 | tar zxf -
# install extension by ID
./code --install-extension <id>

We could also add this to our VSCode desktop module with a coder_script.