refactor images, change heading colors on scroll

[ccchwang]

This PR worked on images and font colors:

• refactored how we pull in images using the Gatsby way of importing them
• added info for the rest of the sections in index.json
• added corresponding colors for the rest of the sections
• added scss to change the color of headings/subheadings/buttons on scroll

[ccchwang]

@leobauza the CI website test is failing again but I think that's okay according to @nhunzaker?

[nhunzaker]

Pretty strange. Let's not worry about it. I wish I knew what was up tho :(

[ccchwang]

@nhunzaker yeah, it sometimes works and sometimes doesn't so I wonder if it's a CI thing? anyways, will keep moving

[codecov-io]

Codecov Report

Merging #464 into master will decrease coverage by 0.21%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #464      +/-   ##
==========================================
- Coverage   93.33%   93.11%   -0.22%     
==========================================
  Files          30       30              
  Lines        1065     1061       -4     
  Branches      199      201       +2     
==========================================
- Hits          994      988       -6     
  Misses         60       60              
- Partials       11       13       +2

Impacted Files	Coverage Δ
packages/microcosm/src/domain-engine.js	80.88% <0%> (-17.65%)	:arrow_down:
packages/microcosm/src/microcosm.js	93.39% <0%> (-5.67%)	:arrow_down:
packages/microcosm/src/addons/action-form.js
packages/microcosm/src/addons/indexing.js	100% <0%> (ø)
packages/microcosm/src/history.js	77.27% <0%> (+0.75%)	:arrow_up:
packages/microcosm/src/utils.js	95.89% <0%> (+1.36%)	:arrow_up:
packages/microcosm/src/addons/action-button.js	100% <0%> (+3.12%)	:arrow_up:
packages/microcosm/src/action.js	98.29% <0%> (+4.27%)	:arrow_up:
packages/microcosm/src/key-path.js	85.71% <0%> (+7.14%)	:arrow_up:
packages/microcosm/src/effect-engine.js	100% <0%> (+11.11%)	:arrow_up:
... and 2 more

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