Fix hydration warning on docs site

[emmatown]

This fixes the hydration warning from React on the website, it was caused by the toasts. I haven't liked the toasts on the website for a while anyway because they don't feel very websitey so I just replaced them with:

• On the homepage with the copy button for yarn create keystone-app, the copy icon turns into a check icon for a second after copying
• On the links on headings, I just removed the copying behavior so clicking the link is just a link to the heading and nothing more which is what sites generally do

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Name	Status	Preview	Updated
keystone-next-docs	✅ Ready (Inspect)	Visit Preview	Sep 13, 2022 at 3:35AM (UTC)

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