docs: Clarifies text for Feature Dataset Presence Matrix, rename schema file

[pablo-gar]

Changes:

• Clarifies 'Feature Dataset Presence Matrix' specification, it was recently noted by @atolopko-czi that the spec was ambiguous and text was added to clarify it
• The schema minor version is bumped.
• Removes version number from schema file to reduce maintenance cost of other docs referring to it.

[codecov]

Codecov Report

Merging #211 (23fded7) into main (61b655d) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #211   +/-   ##
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  Coverage   82.43%   82.43%           
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  Files          28       28           
  Lines        1560     1560           
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  Hits         1286     1286           
  Misses        274      274           

Flag	Coverage Δ
unittests	82.43% <ø> (ø)

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[bkmartinjr]

LGTM.