[JENKINS-73744] Tweaks to Node.js contributing instructions + renovate fix (#9757)

Co-authored-by: Tim Jacomb <21194782+timja@users.noreply.github.com>
Co-authored-by: Basil Crow <me@basilcrow.com>

.github/renovate.json

@@ -128,7 +128,7 @@
     {
       "customType": "regex",
       "fileMatch": [
-        "war/pom.xml"
+        "pom.xml"
       ],
       "matchStrings": [
         "<node.version>(?<currentValue>.*?)</node.version>"

.gitignore

@@ -79,3 +79,6 @@ node_modules/
 
 # Generated JavaScript Bundles
 jsbundles
+
+# In case someone accidentally runs npm install instead of yarn install
+package-lock.json

CONTRIBUTING.md

@@ -52,6 +52,9 @@ MAVEN_OPTS='--add-opens java.base/java.lang=ALL-UNNAMED --add-opens java.base/ja
 
 ### Running the Yarn frontend build
 
+> [!TIP]
+> If you already have Node.js installed, you do not need to change your path. Start using `yarn` by enabling [Corepack](https://yarnpkg.com/corepack) with `corepack enable`, if it isn't already; this will add the `yarn` binary to your PATH.
+
 To run the Yarn frontend build, after [building the WAR file](#building-the-war-file), add the downloaded versions of Node and Yarn to your path:
 
 ```sh
@@ -74,7 +77,7 @@ On one terminal, start a development server that will not process frontend asset
 MAVEN_OPTS='--add-opens java.base/java.lang=ALL-UNNAMED --add-opens java.base/java.io=ALL-UNNAMED --add-opens java.base/java.util=ALL-UNNAMED' mvn -pl war jetty:run -Dskip.yarn
 ```
 
-Open another terminal and start a [webpack](https://webpack.js.org/) dev server, after [adding Node and Yarn to your path](#running-the-yarn-frontend-build):
+Open another terminal and start a [webpack](https://webpack.js.org/) dev server, after [optionally adding Node and Yarn to your path](#running-the-yarn-frontend-build):
 
 ```sh
 yarn start
@@ -108,13 +111,13 @@ To automatically fix backend issues, run:
 mvn spotless:apply
 ```
 
-To view frontend issues, after [adding Node and Yarn to your path](#running-the-yarn-frontend-build), run:
+To view frontend issues, after [optionally adding Node and Yarn to your path](#running-the-yarn-frontend-build), run:
 
 ```sh
 yarn lint
 ```
 
-To fix frontend issues, after [adding Node and Yarn to your path](#running-the-yarn-frontend-build), run:
+To fix frontend issues, after [optionally adding Node and Yarn to your path](#running-the-yarn-frontend-build), run:
 
 ```sh
 yarn lint:fix