cleanup gsTemplates

[hverhelst]

Since gsModule is often used as a template repository, it should have a logical directory structure.
In my experience, the gsTemplates/ directory in gsModule does not fit the design of submodules. Instead, we usually go for putting everything in src/.
Therefore, this PR removes gsTemplates/ from gsModule, aiming to force people to rely on src and making the template cleaner.

[Copilot]

Pull request overview

This PR cleans up the gsModule template repository structure by removing the legacy gsTemplates/ directory usage and consolidating template code under src/, aligning with how submodules are typically organized in this ecosystem.

Changes:

• Add src/gsClass.h as the new location for the template example class header.
• Update the example to include the header from <gsModule/gsClass.h> and remove the old gsTemplates-based includes/usage.
• Update CMakeLists.txt to collect headers/sources/templates from src/ only (dropping gsTemplates/).

Reviewed changes

Copilot reviewed 2 out of 5 changed files in this pull request and generated no comments.

File	Description
src/gsClass.h	Introduces the example class header under src/ to replace the previous templates location.
examples/helloSubmodule.cpp	Updates includes and example usage to match the new header location.
CMakeLists.txt	Removes file collection from gsTemplates/ and consolidates collection to src/.

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