[docs] change Getting started with Julia to use REPL blocks

[odow]

I'll post a preview link once it finishes, but I think we started off too fast with our Julia tutorial.

• We ever explained what the REPL was, or how to access it
• We used a macro in the first example
• It wasn't obvious what parts are code and what parts are output

Preview:

• https://jump.dev/JuMP.jl/previews/PR3077/tutorials/getting_started/getting_started_with_julia/
• https://jump.dev/JuMP.jl/previews/PR3077/JuMP.pdf

[codecov]

Codecov Report

Base: 97.63% // Head: 97.63% // No change to project coverage 👍

Coverage data is based on head (96223a7) compared to base (ac47aca).
Patch has no changes to coverable lines.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3077   +/-   ##
=======================================
  Coverage   97.63%   97.63%           
=======================================
  Files          32       32           
  Lines        4305     4305           
=======================================
  Hits         4203     4203           
  Misses        102      102           

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