Restructure docs to separate Docker and Singularity

[adelavega]

Closes #827

May want to add more detail on how HPC execution works (i.e. SLURM, etc...) but I think that's out of the scope for now.

I restructured the docs to separate Singularity and Docker, as I think this is easier to read for users with only a single environment use case.

[codecov-commenter]

Codecov Report

Merging #1000 (15bd2b9) into master (37e93c5) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1000   +/-   ##
=======================================
  Coverage   78.34%   78.34%           
=======================================
  Files          63       63           
  Lines        3247     3247           
=======================================
  Hits         2544     2544           
  Misses        703      703           

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[cypress]

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Test summary

6 • 1 • 0 • 0 • 0

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Run details

Project	Neuroscout
Status	Failed
Commit	f5fdcc3 ℹ️
Started	Oct 27, 2021 6:21 PM
Ended	Oct 27, 2021 6:22 PM
Duration	01:37 💡
OS	Linux Ubuntu - 20.04
Browser	Electron 87

View run in Cypress Dashboard ➡️

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Failures

	cypress/integration/analysis_builder.spec.js		1
1	Analysis Builder > analysis builder

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