Fix table of contents highlighting glitch

[delucis]

Description

• Fixes Table of Contents Glitch #1448
• See Table of Contents Glitch #1448 (comment) for full details. TL;DR — larger heading top margins increased the likelihood of the table of contents intersection observer falling between elements.
• By increasing the height of the zone we watch for intersections, we can avoid this (unless someone adds content with more extreme top margins, at some point this could still break).

[changeset-bot]

🦋 Changeset detected

Latest commit: 9d0f935

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@astrojs/starlight	Patch

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Name	Status	Preview	Updated (UTC)
starlight	✅ Ready (Inspect)	Visit Preview	Feb 5, 2024 8:23pm

1 Ignored Deployment

Name	Status	Preview	Updated (UTC)
starlight-i18n	⬜️ Ignored (Inspect)		Feb 5, 2024 8:23pm

[astrobot-houston]

size-limit report 📦

Path	Size
/index.html	5.22 KB (-0.02% 🔽)
/_astro/*.js	21.54 KB (-0.02% 🔽)
/_astro/*.css	13.14 KB (0%)

[TheOtterlord]

Thanks, this looks great! Tested on a few resolutions and pages, and everything worked as expected!

[SatanshuMishra]

This is great, Nice job @delucis!