docs: Update template README.md (JavaScript, TypeScript) (#28003)

Use npx to run project-specific cdk, not a globally installed version.

This ensures everybody working on the project uses the same version of the command-line interface.

With the previous instructions, if anybody had `cdk` installed globally, e.g., via `npm install --global aws-cdk` or `homebrew install aws-cdk`, they would run that version. It seems desirable that everybody working on a CDK app uses the same version of the command-line interface, as specified in `package.json` (or `package-lock.json`).

Example:

Given a `package.json` which specifies `"aws-cdk": "2.104.0"` and a globally installed `cdk`, this can happen:

```shell
$ cdk --version
2.108.1 (build 2320255)

$ npx cdk --version
2.104.0 (build 3b99abe)
```


----

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packages/aws-cdk/lib/init-templates/app/javascript/README.md

@@ -7,6 +7,6 @@ The `cdk.json` file tells the CDK Toolkit how to execute your app. The build ste
 ## Useful commands
 
 * `npm run test`         perform the jest unit tests
-* `cdk deploy`           deploy this stack to your default AWS account/region
-* `cdk diff`             compare deployed stack with current state
-* `cdk synth`            emits the synthesized CloudFormation template
+* `npx cdk deploy`       deploy this stack to your default AWS account/region
+* `npx cdk diff`         compare deployed stack with current state
+* `npx cdk synth`        emits the synthesized CloudFormation template

packages/aws-cdk/lib/init-templates/app/typescript/README.md

@@ -9,6 +9,6 @@ The `cdk.json` file tells the CDK Toolkit how to execute your app.
 * `npm run build`   compile typescript to js
 * `npm run watch`   watch for changes and compile
 * `npm run test`    perform the jest unit tests
-* `cdk deploy`      deploy this stack to your default AWS account/region
-* `cdk diff`        compare deployed stack with current state
-* `cdk synth`       emits the synthesized CloudFormation template
+* `npx cdk deploy`  deploy this stack to your default AWS account/region
+* `npx cdk diff`    compare deployed stack with current state
+* `npx cdk synth`   emits the synthesized CloudFormation template