Add more docs about color callbacks

[cmdcolin]

Makes a note about how a custom function for jexl color callbacks is a little different from the jexl function for feature details, and adds a link to FAQ

Also makes the indentation a little less on FAQ, so the sidebar/TOC works better

[cmdcolin]

Also, explains that the "small plugins" that you create with this method do not have to use jbrowse-plugin-template

[codecov]

Codecov Report

Merging #3255 (bd283b9) into main (5eaa86a) will decrease coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #3255      +/-   ##
==========================================
- Coverage   59.41%   59.39%   -0.03%     
==========================================
  Files         674      674              
  Lines       28782    28782              
  Branches     7008     7008              
==========================================
- Hits        17101    17094       -7     
- Misses      11406    11413       +7     
  Partials      275      275              

Impacted Files	Coverage Δ
...svg/src/SvgFeatureRenderer/components/Segments.tsx	85.71% <0.00%> (-7.15%)	⬇️
...src/SvgFeatureRenderer/components/FeatureLabel.tsx	77.50% <0.00%> (-5.00%)	⬇️
...gins/svg/src/SvgFeatureRenderer/components/util.ts	91.83% <0.00%> (-4.09%)	⬇️
...FeatureRenderer/components/ProcessedTranscript.tsx	87.32% <0.00%> (-1.41%)	⬇️
packages/core/util/layouts/GranularRectLayout.ts	86.86% <0.00%> (-0.43%)	⬇️

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