Combine redundant sections about built-in class reference

**Your Godot version:**
4.4
**Issue description:**

The [First look at Godot's interface](https://docs.godotengine.org/en/latest/getting_started/introduction/first_look_at_the_editor.html) page ends with the [Integrated class reference](https://docs.godotengine.org/en/latest/getting_started/introduction/first_look_at_the_editor.html#integrated-class-reference) section. The next page, [Learning new features](https://docs.godotengine.org/en/latest/getting_started/introduction/learning_new_features.html), starts with a section about how to read the manual and class reference, [Making the most of this manual](https://docs.godotengine.org/en/latest/getting_started/introduction/learning_new_features.html#making-the-most-of-this-manual).

Additionally there is a third place this is documented [How to read the Godot API](https://docs.godotengine.org/en/latest/tutorials/scripting/how_to_read_the_godot_api.html), in the scripting section.

They each make sense within their respective pages, but it reads kind of strangely and redundantly if you read the whole Getting Started sequence in order. These should maybe be combined in some way. I'd suggest shortening the section in `First look`, expanding the section in `Learning new features`, and perhaps linking to `How to read the Godot API` from one of the two. I think for a *first look* at an editor, we don't necessarily need to describe extensively how the builtin class reference works.

And the dedicated documentation for what shortcuts are available to access the builtin class reference should not only live in the Getting Started section, since we should treat that as a somewhat optional introductory sequence that you usually shouldn't be going back to for *reference*, IMO.

Perhaps we should write or expand a section that exists somewhere in the Manual rather than the Getting Started section on how the builtin class reference works.

**URL to the documentation page (if already existing):**

https://docs.godotengine.org/en/latest/getting_started/introduction/first_look_at_the_editor.html#integrated-class-reference
https://docs.godotengine.org/en/latest/getting_started/introduction/learning_new_features.html#making-the-most-of-this-manual