fix: resolve incorrect link activation for first and last sections

[Vardhaman619]

In documentation "On this page" section :

1. The first section link was not re-activating when scrolling back to the top after navigating down the page.
2. The last section link was not activating when scroll to bottom.

Before:
before video

before.webm

After:
after-(first-and-last-link-working)-video

after-.first-and-last-links-working.webm

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[vercel]

@Vardhaman619 is attempting to deploy a commit to the Svelte Team on Vercel.

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[Vardhaman619]

@dummdidumm Could you please take a look at this pull request when you have a chance?

[dummdidumm]

thank you!