Adds Application, Configuration, Secrets and Deployment

Why is this change being made?

Related to: Add glossary to discuss deployments (gitlab-org/gitlab#403316 - closed)

This MR adds the following terms:

• Application
• Configuration
• Secrets
• Deployment

This note from Shinya from the previous MR might be helpful to quickly understand the relationships:

Shinya: It sounds like Class is "Application" and Instance is "Environment".

Viktor: Yes! Let's keep this in mind as we add more terms. I don't know how long we will be able to use the object oriented paradigms to speak about delivery, but this sentence captures much better than I could have ever tried to express myself.

The Environment runs a specific instance of an Application. Specific in terms of its Configuration (constructor parameters in object oriented lingo).

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