Get docs to deploy / tab to 4 space

[JeffFessler]

The main change here is to Documenter.yml to try to get "stable" docs to deploy.
In the meantime, changing many tabs to 4 spaces too. No code changes.

[codecov]

Codecov Report

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

Coverage data is based on head (22921e8) compared to base (5c5fe34).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #125   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           36        36           
  Lines         1293      1283   -10     
=========================================
- Hits          1293      1283   -10     

Impacted Files	Coverage Δ
src/regularize/diffl.jl	100.00% <ø> (ø)
src/regularize/diffs.jl	100.00% <ø> (ø)
src/utility/eql_root.jl	100.00% <ø> (ø)
src/utility/map_many.jl	100.00% <ø> (ø)
src/utility/rmsd100.jl	100.00% <ø> (ø)
src/algorithm/general/ncg.jl	100.00% <100.00%> (ø)
src/algorithm/general/ogm_ls.jl	100.00% <100.00%> (ø)
src/algorithm/general/pogm_restart.jl	100.00% <100.00%> (ø)
src/algorithm/general/poweriter.jl	100.00% <100.00%> (ø)
src/fbp/rotate2d.jl	100.00% <100.00%> (ø)
... and 17 more

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