Rewrite manual/variables.md

[odow]

I think there's a lot of improvements that could be made to the Variables page in the manual:

• The introduction is confusing, and front-loads a lot of terminology
• We take too long to get to different parts. We should start by example, then go into detail
• The organization could be improved. More level-2 headers should be "How to" type queries so they are easier to navigate. Currently, we have "Variable names" as a ##, and things like "How to query a variable by name" are buried unless you read the entire docs.

[codecov]

Codecov Report

Merging #2798 (bc14945) into master (744888c) will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2798   +/-   ##
=======================================
  Coverage   94.26%   94.26%           
=======================================
  Files          43       43           
  Lines        5524     5525    +1     
=======================================
+ Hits         5207     5208    +1     
  Misses        317      317           

Impacted Files	Coverage Δ
src/Containers/macro.jl	100.00% <0.00%> (ø)

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