feat: On this page mobile

[PuruVJ]

Adds a OnThisPage right under the heading. Related: sveltejs/site-kit#165

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[dummdidumm]

On this page on desktop looks weird now with this change for some reason

[PuruVJ]

Let's not merge it now then, it needs more brainstorming

[geoffrich]

sveltejs/site-kit#170 fixes a bug with this that would be good to address before merging