Add Using artifact repositories in a restricted environment examples in the Mounting Secrets procedure

We moved Using artifact repositories in a restricted environment chapter out of Dev Spaces 3.0 - https://www.eclipse.org/che/docs/che-7/end-user-guide/using-artifact-repositories-in-a-restricted-environment/ , replacing it by a generic chapter, Using credentials and configurations in workspaces.

The absence of examples in the new docs is preventing QE from testing.

Add examples in the Mounting Secrets procedure to cover the cases previously covered by the Using artifact repositories in a restricted environment chapter.

Mounting secrets:

Example 1: Using Maven artifact repositories

Rename the current example already covering the case.

Example 2: Using Gradle artifact repositories

Add a ~/.gradle/init.gradle secret

Example 3: Using NuGet artifact repositories

Add a /projects/nuget.config file

Mounting ConfigMaps

Example 1: Using Python artifact repositories

Configure the PIP_INDEX_URL and PIP_CERT environment variables.

Example 2: Using Go artifact repositories

Configure the GOPROXY environment variables.

Example 3: Using npm artifact repositories

Configure the NPM_CONFIG_REGISTRY and NODE_EXTRA_CA_CERTS environment variables.