Add a short section that explains how to share a profile to the homepage

[canova]

For some contributors, it's not clear to how to share a performance profile with others, and they think that /from-browser profile urls are enough to share.

As also :smaug mentioned in our matrix channel, it could be god to explain how to share the profile with the "upload local profile" button. We can possibly add this after the text explaining how to capture a profile.

We can also add a link to our documentation section that explains the sharing behavior: https://profiler.firefox.com/docs/#/./guide-getting-started?id=share-a-profile

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