[web] Cleanup node_modules before installing packages.

This fixes build errors when node modules were updated.
It should not be costly as yarn caches downloads.

scripts/jenkins/gcc.groovy

@@ -26,7 +26,7 @@ image.pull()
 image.inside(defaultDockerArgs) {
     sh 'cd ogs && git lfs pull'
     stage('Install prerequisites Web') {
-        sh 'cd ogs/web && npm install && sudo -H pip install -r requirements.txt'
+        sh 'cd ogs/web && rm -rf node_modules && yarn && sudo -H pip install -r requirements.txt'
     }
     stage('Configure (Linux-Docker)') {
         configure.linux(cmakeOptions: defaultCMakeOptions + webCMakeOptions, script: this)

web/content/docs/devguide/getting-started/introduction.md

@@ -9,5 +9,3 @@ aliases = [ "/docs/devguide/" ] # First devguide page
 +++
 
 In this help section you will find everything related to development. Please walk through the Getting Started-section **step by step**. At the end you will have the latest OGS source code, the OGS finite element simulator compiled and everything you need to start developing for OGS! This first section of the developer guide will give you just a brief introduction, make sure to read the more advanced topics after you have familiarized yourself with the basics.
-
-{{< figure src="/docs/devguide/getting-started/test.jpg" class="img-responsive" >}}