DOC: Minor doc cleanup because of Panel removal

[topper-123]

"Other data structures..." is not usuful anymore, as Panel and Panel4D are have been removed for the code base,

Plus other small stuff.

[codecov]

Codecov Report

Merging #26638 into master will decrease coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #26638      +/-   ##
==========================================
- Coverage   91.88%   91.87%   -0.01%     
==========================================
  Files         174      174              
  Lines       50692    50692              
==========================================
- Hits        46576    46572       -4     
- Misses       4116     4120       +4

Flag	Coverage Δ
#multiple	90.41% <ø> (ø)	⬆️
#single	41.8% <ø> (-0.09%)	⬇️

Impacted Files	Coverage Δ
pandas/io/gbq.py	78.94% <0%> (-10.53%)	⬇️
pandas/core/frame.py	97% <0%> (-0.12%)	⬇️

---

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[codecov]

Codecov Report

Merging #26638 into master will decrease coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #26638      +/-   ##
==========================================
- Coverage   91.88%   91.87%   -0.01%     
==========================================
  Files         174      174              
  Lines       50692    50692              
==========================================
- Hits        46576    46572       -4     
- Misses       4116     4120       +4

Flag	Coverage Δ
#multiple	90.41% <ø> (ø)	⬆️
#single	41.8% <ø> (-0.09%)	⬇️

Impacted Files	Coverage Δ
pandas/io/gbq.py	78.94% <0%> (-10.53%)	⬇️
pandas/core/frame.py	97% <0%> (-0.12%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 8d124ea...33bcf73. Read the comment docs.