Checks to be made across lessons

[cforgaci]

I started collecting feedback from lesson-level PRs that apply across lessons in the form of a checklist:

• no inline code is used in callout headings #9

  • Lesson 1
  • Lesson 2
  • Lesson 3
  • Lesson 4

• questions are written in first person

• the same phrasing is used for objectives

• determine stars for all challenges and list them in one place #10

• harmonise naming of exercises/challenges and precede all challange callout headings with Challenge:

  • Lesson 1
  • Lesson 2
  • Lesson 3
  • Lesson 4

Good to have, but not essential:

• all code chunks should have a unique name (unnamed chunks are auto-named by default)
  • Lesson 1
  • Lesson 2
  • Lesson 3
  • Lesson 4