add support for loading env vars with configurable prefixes

[miniscruff]

Closes #372

Also through in some doc updates

Check the following

• Keep 100% code coverage
• Be properly formatted
• Documentation changes are included
• Include a change file if expected

Additional context
Any additional info that might help get your pull request merged.

[codecov]

Codecov Report

Base: 100.00% // Head: 100.00% // No change to project coverage 👍

Coverage data is based on head (169c1a4) compared to base (0a80659).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #386   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           14        14           
  Lines         1569      1620   +51     
=========================================
+ Hits          1569      1620   +51     

Impacted Files	Coverage Δ
cmd/next.go	100.00% <ø> (ø)
core/change.go	100.00% <ø> (ø)
core/templatecache.go	100.00% <ø> (ø)
cmd/batch.go	100.00% <100.00%> (ø)
cmd/gen.go	100.00% <100.00%> (ø)
cmd/init.go	100.00% <100.00%> (ø)
core/config.go	100.00% <100.00%> (ø)
core/utils.go	100.00% <100.00%> (ø)

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