Updated header in app.py

[Spyhuntr]

Changed the layout of the header so it's more responsive across multiple screen sizes. Also added a bootstrap logo for branding sake :D

[AnnMarieW]

Hi @Spyhuntr

Thanks for the pull request - your changes are a huge improvement to the app design. And it looks so much better on a smaller screen!

A couple minor comments:

• Let's use a Plotly logo rather than Bootstrap, since this project uses components from other libraries as well.
• Looks like there's room to put "Example Index" in the nav bar in the small screens rather than DEI

I'm looking forward to seeing your ideas on how to jazz up the "Welcome to the Dash Example Index" box too, if you are up for more design contributions.

[Spyhuntr]

Made the changes from the comments.

[AnnMarieW]

Thanks the the contribution :woman_dancing: