[Doc] Improvements to the Setup Section in `CONTRIBUTING.md`

[starry48]

I've noticed there are few areas in the Setup section of CONTRIBUTING.md that could be clarified or updated to become more beginner-friendly, especially those new to Poetry or Textual. Here are the suggested improvements:

1. Poetry Installation Link Redirecting to the instructions

• The current link in the first step to install poetry takes users to Poetry's homepage rather than the installation instructions
• Suggestions: Update it brings users directly to the instructions page of the website

2. poetry shell command needs prerequisites installed

• Running the poetry shell command assumes users have the packages installed for the command to work, which may not be true to beginners
• Suggestion: Another command that doesn't require these installations but still opens the virtual environment is poetry env activate

3. Clarify textual --version requires poetry run

• Newcomers to the project might miss that they need the poetry run prefix command
• Suggestion: Add poetry run to the textual --version command

4. Adding how to Run examples

• After setup, beginners might wonder how to test an example script
• Suggestion: Include another step which explains to use the poetry run python <SCRIPT_NAME.py> to execute an example in the examples folder