Scroll Depth Release

[RobertJoonas]

Changes

This PR can be merged once the scroll depth script has been collecting data for a couple of days already. Will be keeping it as a draft until then.

Companion PRs:

1. docs: scroll depth docs#573
2. website blog post: Create Blog “2025-01-17-the-easiest-scroll-depth-tracking-ever-–-exclusively-in-plausible” website#735

This PR:

• updates changelog about the new features (also about the bfcache fix that is currently under the pageleave extension)
• Adds info paragraphs with documentation links into the "Add goal" form for both pageview and scroll goals
• Makes the scroll_depth metric available in the public API

Tests

• Automated tests have been added

Changelog

• Entry has been added to changelog

Documentation

• PRs open as mentioned above

Dark mode

• The UI has been tested both in dark and light mode