vsce docs update: file structure overview

Created by: abeatrix

This PR adds an overview of the file structure for the VS Code Extension source code.

I've also simplified the name of a folder.

Test plan

❯ yarn test-integration
yarn run v1.22.17
warning @sourcegraph/vscode@2.2.4: The engine "vscode" appears to be invalid.
$ TS_NODE_PROJECT=tests/tsconfig.json mocha --parallel=$CI --retries=2 ./tests/**/*.test.ts


  VS Code extension
Downloading VS Code 1.68.0 from https://update.code.visualstudio.com/1.68.0/darwin-arm64/stable
Downloading VS Code [==============================] 100%Downloaded VS Code 1.68.0 into /Users/b/Dev/sourcegraph/client/vscode/.vscode-test/vscode-darwin-1.68.0
Starting VS Code {
  verbose: false,
  vscodeExecutablePath: '/Users/b/Dev/sourcegraph/client/vscode/.vscode-test/vscode-darwin-1.68.0/Vis'... 43 more characters,
  userDataDirectory: '/var/folders/g1/f24vdsfn2mb1z2_wwyl5vygh0000gn/T/vsce78yWVY',
  extensionsDirectory: '/var/folders/g1/f24vdsfn2mb1z2_wwyl5vygh0000gn/T/vsceD159jC'
}
VSCode started in debug mode on http://127.0.0.1:29378
    ✓ works (9969ms)


  1 passing (33s)

✨  Done in 41.00s.

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