Document configuration for corporations

[romain-pm]

Following the first iterations of the JavaScript modules training, we need to improve the documentation to:

• identify issues faster: verify access to npmjs, jahia nexus, jahia store, ability to run docker, ability to install yarn, ...
• add information about configuration that can help (how to use a different nexus and npmjs repos)

[fbourasse]

Some things that were useful:

• How to change the default registry URL of npm: npm config set --global "registry" https://NEXUS_URL/nexus/repository/registry.npmjs.org/
• How to determine where are the modules of npm?npm root -g
• How to install yarn without using an installer: npm i -g @yarnpkg/cli-dist
• How to change the default registry URL of yarn: yarn config set --home npmRegistryServer https://NEXUS_URL/nexus/repository/registry.npmjs.org/