Inconsistent instructions regarding compiling custom export templates per platform

**Your Godot version:**
3.0+

**Issue description:**
First, look at the various pages for [compiling Godot](https://docs.godotengine.org/en/latest/development/compiling/index.html) for different platforms, and notice that some pages describe the entire build cycle, while others (mainly macOS and iOS #2739) pretty much lack the instructions on how to compile and/or use custom export templates (how to set them via editor's export presets etc.).

I think it would be nice to have a dedicated page describing the process of compiling export templates for Godot *in general* as an introductory material without relying on the platform-specific instructions. The main takeaway should be that there's a difference between (compiling) an editor and the export templates, and this should be clearly emphasized.

At the very least, each of the platform-specific instructions should likely just point to:
* [Introduction to the buildsystem - Export templates](https://docs.godotengine.org/en/latest/development/compiling/introduction_to_the_buildsystem.html#export-templates).

while also eliminating some duplicate information as in #3469.

Some proposals and issues demonstrating the struggle of new users understanding these two processes:
* godotengine/godot#35299, #1029:
> This step should be mentioned in Custom modules in C++ documentation, it should at least link to the export templates documentation as it's really confusing when you follow the tutorial and your custom module doesn't work when exported.
* godotengine/godot-proposals#288.
* Xrayez/godot-imagetools#2

Thoughts?

**URL to the documentation page (if already existing):**
See above, but mostly lack of info for [Compiling for macOS](https://docs.godotengine.org/en/latest/development/compiling/compiling_for_osx.html), and likely some others.