Guide Content list element is not highlighting Last index element even we are reading last content

### Is this a regression?

- [ ] Yes, this behavior used to work in the previous version

### The previous version in which this bug was not present was

_No response_

### Description

I am going through https://material.angular.io/guide/schematics
and I noticed following things
1) when I reading left content by content it highlights current topic name in right side list
but its not working for last content
- when i am reding last content **Material 3 Theme schematic** but right side highlighter showing only on **Drag and Drop schematic** instead of **Material 3 Theme schematic**

### Reproduction

I am going through https://material.angular.io/guide/schematics
and I noticed following things
1) when I reading left content by content it highlights current topic name in right side list
but its not working for last content
- when i am reding last content **Material 3 Theme schematic** but right side highlighter showing only on **Drag and Drop schematic** instead of **Material 3 Theme schematic**

### Expected Behavior

when I am reading last content in list then only last element is not highlighted in **Guide Content** list

### Actual Behavior

when I am reading last content in list then only last element is should be highlighted in **Guide Content** list

### Environment

打开 https://material.angular.io/guide/schematics 上的 Guide Schematics 页面，并阅读到最后的“Material 3 Theme schematic”部分来重现该问题。跟踪 Guide Content 的高亮行为，并验证当最后一个条目的内容可见时，该条目会被高亮显示，同时不改变之前条目的高亮状态。

由索引模型根据 Issue 内容生成。