feat: third lesson about DevTools

[honzajavorek]

Porting this lesson and this lesson:

• I deliberately didn't include anything about cleaning data or iterating over the data, because it would be unnecessary load of JavaScript for a Python dev, for no obvious reason. They'll learn everything in the following lessons. I focused only on stuff which can be considered as ad-hoc inspection of the website, which is useful even to Python devs.
• I tried to unify the style with the rest of the course. I wrote the stuff with my own words, and added standard polishing I usually do (dictionary, proofreading by AI, etc.)
• I added several screenshots.
• I added exercises.