Restructure Lesson 3: Remove WebApp, Smaller Modules, Integrate Exercises…

[thoward]

Closes #64, Closes #65.

This PR restructures Lesson 3 to remove the section on the web app, have smaller modules, interleaves the exercises with the explanation, and uses the new uniform lesson structure.

I kept the majority of the original content, but removed the exercise on sensuctl dump mostly for brevity's sake and because it felt like a use case that was beyond "absolute beginner".

Quantitative Analysis Using F-K Calculator (code samples removed):

• Flesh-Kincaid Grade Level: 8.3
• Flesch Reading Ease Score: 47.7
• Reading Level: College ( Difficult to read )
• Average Words per Sentence: 6.8
• Average Syllables per Word: 1.8
• Sentences: 322
• Words: 2181*

*NOTE: The overall length is still longer than I'd prefer, but I think it's within an acceptable limit. Would be good to do a "speed run" on this to see how long it takes to get through it with the new format.

View the rendered Markdown here: https://github.com/sensu/sensu-go-workshop/blob/381e36b700cce86133f1045d26d96ce9b6bc0c4a/lessons/operator/03/README.md

[thoward]

Aside: I will likely take one more pass through this before merge to add more links. During review if you notice a spot that looks like a good place for a link, please mention it!

Also, the "Lesson X" for Sensu Web App walkthrough still needs to be made, but not what numberin to use. Would like to have a quick chat about options for that.

[calebhailey]

Great job! I love the new structure.

I left several comments with mostly minor nits and only one (!) typo.