Refactor examples_dev.m into smaller, independent examples and move it to the toolbox folder.

[slahoti1]

This PR splits the large examples_dev.m file into smaller, individual example files and moves them into a new gramm/examples folder. The goal is to make examples easier to find, easier to understand, and ensure they ship inside the toolbox package.

What this change includes:

• Refactored "examples_dev.m" into separate example files.
• Added all examples under "gramm/examples/", along with the sample data they need
• Updated the test suite to use these new modular examples
• Have not removed the original "examples_dev.m"

Why this helps

• Examples are easier to navigate
• Users get all examples directly after installing the toolbox
• Each example is focused on a specific functionality, which makes things clearer
• Modular examples also act as quick smoke tests for "gramm" functionality

Issue
Fixes #142