DOC: better explanations of conda env names in install docs

[drammock]

• brings docs in line with new practice of installing into base
• makes contributor docs more explicit about creating new env first when setting up dev environment

[codecov]

Codecov Report

Merging #6747 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #6747   +/-   ##
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  Coverage   89.59%   89.59%           
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  Files         419      419           
  Lines       75837    75837           
  Branches    12422    12422           
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  Hits        67944    67944           
  Misses       5098     5098           
  Partials     2795     2795

[larsoner]

Thanks @drammock