Event Manager Project: Add lesson note about API key security

[SumonGFC]

Because

This would add a lesson note about why API key security is important in the Event Manager Project, which in its current state does not offer any information about hiding our keys.

This PR

• Adds a lesson note on line 746 to line 758

Issue

Closes #28246

Additional Information

Pull Request Requirements

• [x] I have thoroughly read and understand The Odin Project curriculum contributing guide
• [x] The title of this PR follows the location of change: brief description of change format, e.g. Intro to HTML and CSS lesson: Fix link text
• [x] The Because section summarizes the reason for this PR
• [x] The This PR section has a bullet point list describing the changes in this PR
• [x] If this PR addresses an open issue, it is linked in the Issue section
• [x] If any lesson files are included in this PR, they have been previewed with the Markdown preview tool to ensure it is formatted correctly
• [x] If any lesson files are included in this PR, they follow the Layout Style Guide