feat: On this page mobile

[PuruVJ]

Adds a OnThisPage right under the heading. Related: https://github.com/sveltejs/site-kit/pull/165

Before submitting the PR, please make sure you do the following

• [ ] It's really useful if your PR references an issue where it is discussed ahead of time. In many cases, features are absent for a reason. For large changes, please create an RFC: https://github.com/sveltejs/rfcs
• [x] Prefix your PR title with feat:, fix:, chore:, or docs:.
• [x] This message body should clearly illustrate what problems it solves.
• [ ] Ideally, include a test that fails without this PR but passes with it.

Tests and linting

• [ ] Run the tests with pnpm test and lint the project with pnpm lint

[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]

https://github.com/sveltejs/site-kit/pull/170 fixes a bug with this that would be good to address before merging