Add Webinar for Fall 2024

[amazingrando]

This PR does the following:

• Adds a webinar page
• Updates the theme switcher in the header (dark/light mode toggle)

Functional Testing

• [x] Visit https://deploy-preview-132--emulsify-website.netlify.app/webinars/fall-2024 and
• [x] Confirm the site works on small and large screens
• [x] Confirm that light and dark modes work
• [x] Use the theme switcher toggle in the header to confirm that dark/light/system modes are working.
• [ ] Submit your name and email to the form. Confirm that the form confirms that you are signed up.
• [ ] Visit the spreadsheet and confirm that the information you entered into the form has been added.

[amazingrando]

@mariannuar Thank you for catching those issues! This video shows you the fixes: https://www.loom.com/share/c17dc767ff3d4fa391e307fd3ae8f370?sid=c9bf5d08-1b12-4ef3-9bb6-b8b63ed0502a

The webinar page is needed, so I'm merging this PR.