adding vignette urls to docs

[IndrajeetPatil]

• follows styling guidelines in vignettes
• follows 80-character rule for roxygen docs

[codecov-commenter]

Codecov Report

Merging #177 (9f84416) into develop (3ce8490) will not change coverage.
The diff coverage is 50.00%.

@@           Coverage Diff            @@
##           develop     #177   +/-   ##
========================================
  Coverage    48.41%   48.41%           
========================================
  Files           55       55           
  Lines         2551     2551           
========================================
  Hits          1235     1235           
  Misses        1316     1316           

Impacted Files	Coverage Δ
R/plot-boxwhisker.R	0.00% <ø> (ø)
R/plot-pkratio.R	93.33% <ø> (ø)
R/themes.R	0.00% <0.00%> (ø)
R/utilities-aesthetics.R	87.80% <100.00%> (ø)

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