add intro and conclusion sections

[ccchwang]

• added the intro and conclusion sections
• refactored pages/index.js and section.scss to accommodate for visual changes in intro/conclusion

[codecov-io]

Codecov Report

Merging #466 into ch/add-images will decrease coverage by 0.46%. The diff coverage is n/a.

@@                Coverage Diff                @@
##           ch/add-images     #466      +/-   ##
=================================================
- Coverage          94.92%   94.46%   -0.47%     
=================================================
  Files                 30       30              
  Lines               1065     1065              
  Branches             199      199              
=================================================
- Hits                1011     1006       -5     
- Misses                47       49       +2     
- Partials               7       10       +3

Impacted Files	Coverage Δ
packages/microcosm/src/install-devtools.js	16.66% <0%> (-83.34%)	:arrow_down:
packages/microcosm/src/history.js	84.09% <0%> (-10.61%)	:arrow_down:
packages/microcosm/src/addons/action-button.js	96.87% <0%> (-3.13%)	:arrow_down:
packages/microcosm/src/action.js	97.43% <0%> (-0.86%)	:arrow_down:
packages/microcosm/src/microcosm.js	95.28% <0%> (+2.83%)	:arrow_up:
packages/microcosm/src/key-path.js	78.57% <0%> (+7.14%)	:arrow_up:
packages/microcosm/src/domain-engine.js	98.52% <0%> (+11.76%)	:arrow_up:
packages/microcosm/src/lifecycle.js	100% <0%> (+33.33%)	:arrow_up:

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