refactor: restructure episode 3

[yld-weng]

Move sections Types control what operations (or methods) can be performed on a given value and Must convert numbers to strings or vice versa when operating on them. together as they are both focus on operations between different types.

Close #600.

[alee]

Thank you for the PR! Though I'm sorry to say that after taking a closer look at the episode with the rearranged key points / sections I'm not convinced this is actually an improvement, despite encouraging people to take a closer look in #600

For clarity, this PR and changeset makes the sections and corresponding key points go follow this thread:

• Every value has a type.
• Use the built-in function type to find the type of a value.
• Strings can be added and multiplied.
• Strings have a length (but numbers don’t).
• Can mix integers and floats freely in operations.
• Variables only change value when something is assigned to them.
• Types control what operations can be done on values.
• Must convert numbers to strings or vice versa when operating on them.

The original:

• Every value has a type.
• Use the built-in function type to find the type of a value.
• Types control what operations can be done on values. then we follow up with the kinds of operations types enable
• Strings can be added and multiplied.
• Strings have a length (but numbers don’t).
• Must convert numbers to strings or vice versa when operating on them. start to delve into type coercion
• Can mix integers and floats freely in operations. continue type coercion
• Variables only change value when something is assigned to them. this does seem a bit out of the blue and starts to scratch the surface of references, mutation, and variable assignment semantics but also seems an important concept to convey to learners

I feel like the original's ordering works fine but am open to thoughts from others who have taken the course or taught the course. Thoughts?

[yld-weng]

Thanks for the review, I'll close this PR for now