The first part of that text (## [Welcome to %{title}](#welcome)) is markdown that creates an h2 element with a link. The link’s href is set to “#welcome”. The %{title} tag is a placeholder that is used by Discourse to substitute your site’s title into the text.
It should be possible to customize this text, but you need to be sure you are using valid markdown. For example, this works:
## [Welcome to My Demo Site](#welcome)
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This also works, it removes the link from the h2 element:
## Welcome to My Demo Site
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