Include code smell under refactoring

[tlylt]

Code smell might be a worth knowing concept when learning about
refactoring.

Let's include it under refactoring so that learners can be aware of the
term and pick up some ideas on how code smells might indicate a
refactoring is overdue.

I have also added a list of links for code smell under reference.

Fixes #107

P.S Hi prof, not sure why the deployment failed, if there is any required changes on my end please feel free to let me know. Thank you.

[damithc]

Thanks for the PR @tlylt Looks like you put in a good amount of effort into this 💯
Given this is textbook, it is not easy for external contributors to add content (beyond smaller bug fixes) -- as there are many different aspects to balance. For example, the depth of the topic should match the other topics, and it should not introduce unnecessary dependencies between topics (as different courses uses different orderings of the topics). I will factor in your proposed content when I add this topic to the book (hopefully, later in the summer). Most likely it will be a shorter version of your proposed contents. I'll leave this PR in view until then.

[tlylt]

Thanks for the PR @tlylt Looks like you put in a good amount of effort into this 💯
Given this is textbook, it is not easy for external contributors to add content (beyond smaller bug fixes) -- as there are many different aspects to balance. For example, the depth of the topic should match the other topics, and it should not introduce unnecessary dependencies between topics (as different courses uses different orderings of the topics). I will factor in your proposed content when I add this topic to the book (hopefully, later in the summer). Most likely it will be a shorter version of your proposed contents. I'll leave this PR in view until then.

Sure thing Prof, thank you for your kind reply!

[damithc]

I've added a light mention of code smells in edca446, and mentioned this PR in the commit message. Adding more detailed version can be considered in a future date. Thanks again for your work in this PR @tlylt