[r] Vignette for new-shape feature

[johnkerl]

Issue and/or context: As tracked on issue #2407 / [sc-51048].

Note that the intended Python and R API changes are all agreed on and finalized as described in #2407.

Changes:

This is the R counterpart to #3294.

Notes for Reviewer:

There will be Academy material as well, in a separate PR.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.27%. Comparing base (d0cbd2d) to head (aa39bfc).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3302      +/-   ##
==========================================
+ Coverage   85.09%   85.27%   +0.17%     
==========================================
  Files          53       53              
  Lines        5577     5574       -3     
==========================================
+ Hits         4746     4753       +7     
+ Misses        831      821      -10     

Flag	Coverage Δ
python	85.27% <100.00%> (+0.17%)	⬆️

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

Components	Coverage Δ
python_api	85.27% <100.00%> (+0.17%)	⬆️
libtiledbsoma	∅ <ø> (∅)