[MRG] update various descriptions to talk about k-mers, not just DNA

[ctb]

This PR adjusts sourmash doc descriptions to emphasize "k-mers" and "biological sequences", not just "DNA sequences".

This PR also edits a number of dib-lab/sourmash references to sourmash-bio/sourmash references.

fixes #2115

[codecov]

Codecov Report

Merging #2137 (7c2f1f0) into latest (8beaaca) will not change coverage.
The diff coverage is 100.00%.

❗ Current head 7c2f1f0 differs from pull request most recent head 51526e7. Consider uploading reports for the commit 51526e7 to get more accurate results

@@           Coverage Diff           @@
##           latest    #2137   +/-   ##
=======================================
  Coverage   84.31%   84.31%           
=======================================
  Files         130      130           
  Lines       15293    15293           
  Branches     2167     2167           
=======================================
  Hits        12895    12895           
  Misses       2095     2095           
  Partials      303      303           

Flag	Coverage Δ
python	91.70% <100.00%> (ø)
rust	65.29% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/sourmash/__init__.py	100.00% <ø> (ø)
src/sourmash/cli/__init__.py	95.74% <100.00%> (ø)

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[mr-eyes]

LGTM!