Mention how to share a profile in the home page

:smaug mentioned in our Matrix channel that quite often people have copy-pasted the url from the location bar instead of using the share button and sharing the permalink. We do have this information in our user documentation but this is not very discoverable. Instead, we should mention how to share a profile in the home page, so people can see how to do it easily.

:smaug's suggestion was adding a text like this after our shortcuts on how to capture a profile:
"...and if you want to share the profile, click 'upload local profile' and copy-paste the generated link"

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-545)