feat: add demo warning to projects

[naomi-lgbt]

Checklist:

• I have read freeCodeCamp's contribution guidelines.
• My pull request has a descriptive title (not a vague title like Update index.md)

Closes freeCodeCamp/freeCodeCamp#51152

Currently playing with a modal approach. To test:

• Clone
• cd apps/personal-portfolio
• npm i
• npm start

I figure a modal that forces campers to accept before interacting with the page might be more attention grabbing.

If we are happy with this approach, I'll:

• Move the script to a .js file in our CDN
• Load the script in the HTML for each of the projects