Add refresher tips for how to do tasks, explanations, minor style changes

[y-almannaee]

This pull request changes the following things on the "Creating the enemy" page under the "Your first 2D game" tutorial:

• Explains how to change scenes, reminds the user to save their new Mob scene
• Adds a tip to remind users how to group a node with its children
• Explains what the Layer and Mask selections do in the CollisionObject2D
• Adds a tip to remind users how to add animation sprites
• Adds a comment for other contributors to update a code section with changes to match the example project
• Adds a tip to remind users how to connect signals to nodes
• Adds a note on how users can learn what an unknown function does in the Script Editor
• Minor style updates, manually wrap lines

Please review some of the additions for verbosity. Thank you!