Check the number of active codespaces before creating a new codespace in Step 3

[Nkevi]

Summary

Following the steps in the course to continuously create new codespace, until step 3, we may reach the maximum number of active codespaces (typically two) and have to stop one of those before creating a new one.

Changes

Revise the instructions to include checking the number of active codespaces and how to stop an active codespace in 3-customize-codespace.md file.

Closes: N/A

Task list

• For workflow changes, I have verified the Actions workflows function as expected.
• For content changes, I have reviewed the style guide.