Some more consistency changes to subgrad_distance

[hajg-ijk]

I updated the docs to actually reflect the correct default exponent (c=2), and replaced instances where 0 and 1.0 occurred with zero and one respectively.
Finally, I added subgrad_distance to the exported functions.

[codecov]

Codecov Report

Merging #32 (507516c) into main (cf4a488) will decrease coverage by 0.06%.
The diff coverage is 100.00%.

❗ Current head 507516c differs from pull request most recent head 254f2f3. Consider uploading reports for the commit 254f2f3 to get more accurate results

@@            Coverage Diff             @@
##             main      #32      +/-   ##
==========================================
- Coverage   95.26%   95.20%   -0.06%     
==========================================
  Files          20       15       -5     
  Lines         359      313      -46     
==========================================
- Hits          342      298      -44     
+ Misses         17       15       -2     

Files Changed	Coverage Δ
src/ManifoldDiff.jl	96.00% <ø> (-0.67%)	⬇️
src/subgradients.jl	100.00% <100.00%> (ø)

... and 5 files with indirect coverage changes

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[kellertuer]

Maybe still bump the version so we can register this patch.

[hajg-ijk]

Right, I totally forgot about that.

[mateuszbaran]

Thanks! I just have one minor comment.