Added Standalone documentation

[cheina97]

This PR adds the documentation about Standalone Applications.

In particular, it contains:

• an explanation about what are Standalone Applications.
• the requirements that a Standalone Application has to respect in order to be deployed successfully.
• an example that describes step by step how to create a Standalone Application and deploy it.
• the documentation about code-server
• how to customize and deploy code-server

[kingmakerbot]

Hi @cheina97. Thanks for your PR.

I am @kingmakerbot.
You can interact with me issuing a slash command in the first line of a comment.
Currently, I understand the following commands:

• /rebase: Rebase this PR onto the master branch
• /merge: Merge this PR into the master branch
• /hold: Adds hold label to prevent merging with /merge
• /unhold: Removes the hold label to allow merging with /merge
• /deploy-staging: Deploy a staging environment to test this PR (the build-all flag enables user environments building)
• /undeploy-staging: Manually undeploy the staging environment

Make sure this PR appears in the CrownLabs changelog, adding one of the following labels:

• kind/breaking: 💥 Breaking Change
• kind/feature: 🚀 New Feature
• kind/bug: 🐛 Bug Fix
• kind/cleanup: 🧹 Code Refactoring
• kind/docs: 📝 Documentation

[giorio94]

Sorry for the late review.

A few comments inline, but overall looks mostly good to me.

[giorio94]

/lgtm

Just a couple of final comments, then feel free to squash and merge

[cheina97]

/merge