Docs: Create guidelines and checklist for lessons and practices

We need to create a comprehensive guidelines document that will serve as a reference when writing or reviewing lessons and practices in the Learn GDScript app.

This would help to maintain consistency and quality across all content in the app. It could actually go beyond and be used for GDPractices in general. But a short version tailored to Learn GDScript wouldn't hurt either.

This document should include two main sections with checklists:

**Lesson guidelines**

The lesson guidelines should cover:

- Clarity: Ensure explanations are clear and understandable for ESL students
- Engagement: Include playful elements and celebrations when appropriate to keep learning fun
- Offloading content: Consider what extra information, recaps, study guides, cheat sheets could be written in the GDLibrary and linked from lessons
- Setting expectations: Add checkpoints to help students understand what they should know (or not know yet) at different stages, in particular, at the end of lessons). Example: "Don't worry if it's not clear yet, it's a tricky concept to wrap your head around, you'll need to revisit it multiple times to understand it fully."

**Practice guidelines**

The practice guidelines should ensure that:

- All relevant variables needed for the practice are clearly mentioned.
- Goals are clear. The practice objectives should be stated clearly.
- The execution/running context is clear. Explain how the student's code will be called and tested by the app.
- Visuals are clear. The visual console (where we display running game code) has clear labels and helpful information to guide students. Wxample of missing clarity: train tracks practice for negative array index access.
- Difficulty is progressive. Practices build on concepts taught in lessons and increase in difficulty gradually.