Add Actorization challenge banner to homepage

[webrdaniel]

Add a banner to the docs homepage that links to https://apify.com/challenge, same as we have on our main homepage.

Figma: https://www.figma.com/design/r6aPCnTKl1Gy3rPytKgndK/Documentation?node-id=2123-36798&p=f
Mobile design of the banner: https://www.figma.com/design/Cz4PCMF1jkkvQMRvQroyal/Web-library?node-id=3812-11181&m=dev

[jbartadev]

Additional notes:

• Center the hero section
• Make the banner same on mobile as on the web