Add a brief example of concatenating frames

[jeremykubica]

Add some documentation for how concatenation works.

[review-notebook-app]

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.19%. Comparing base (f592cac) to head (0d7d1df).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #372   +/-   ##
=======================================
  Coverage   98.19%   98.19%           
=======================================
  Files          19       19           
  Lines        1880     1880           
=======================================
  Hits         1846     1846           
  Misses         34       34           

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[github-actions]

Before [f592cac]	After [476c884]	Ratio	Benchmark (Parameter)
135M	135M	1	benchmarks.CountNestedBy.peakmem_run
102M	102M	1	benchmarks.NestedFrameAddNested.peakmem_run
106M	106M	1	benchmarks.NestedFrameQuery.peakmem_run
105M	105M	1	benchmarks.NestedFrameReduce.peakmem_run
250M	248M	0.99	benchmarks.AssignSingleDfToNestedSeries.peakmem_run
1.37±0.01ms	1.36±0.01ms	0.99	benchmarks.NestedFrameReduce.time_run
179M	177M	0.99	benchmarks.ReadFewColumnsHTTPS.peakmem_run
1.98±0.1s	1.96±0.01s	0.99	benchmarks.ReadFewColumnsS3.time_run
270M	268M	0.99	benchmarks.ReassignHalfOfNestedSeries.peakmem_run
34.3±0.7ms	33.6±0.5ms	0.98	benchmarks.AssignSingleDfToNestedSeries.time_run

Click here to view all benchmarks.

[dougbrn]

Looks good, thank you!