DOC: Clean up of API doc sections

[datapythonista]

Follow up of the splitting of api.rst in #24462.

• Making better use of the titles and subtitles (in date offsets, the class name, and the properties and methods sections where at the same level for example).
• We were inconsistently using Attributes and Properties, left just properties, as it was the most common one.
• Renamed indices.rst to indexing.rst, I think it makes more sense (index.rst is kind of reserved to the home)

[codecov]

Codecov Report

Merging #24508 into master will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #24508      +/-   ##
==========================================
- Coverage   31.91%    31.9%   -0.02%     
==========================================
  Files         166      166              
  Lines       52400    52428      +28     
==========================================
+ Hits        16722    16725       +3     
- Misses      35678    35703      +25

Flag	Coverage Δ
#multiple	30.29% <ø> (-0.02%)	⬇️
#single	31.9% <ø> (-0.02%)	⬇️

Impacted Files	Coverage Δ
pandas/core/frame.py	27.95% <0%> (-0.42%)	⬇️
pandas/core/dtypes/inference.py	80% <0%> (+0.96%)	⬆️

---

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[codecov]

Codecov Report

Merging #24508 into master will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #24508      +/-   ##
==========================================
- Coverage   31.91%    31.9%   -0.02%     
==========================================
  Files         166      166              
  Lines       52400    52428      +28     
==========================================
+ Hits        16722    16725       +3     
- Misses      35678    35703      +25

Flag	Coverage Δ
#multiple	30.29% <ø> (-0.02%)	⬇️
#single	31.9% <ø> (-0.02%)	⬇️

Impacted Files	Coverage Δ
pandas/core/frame.py	27.95% <0%> (-0.42%)	⬇️
pandas/core/dtypes/inference.py	80% <0%> (+0.96%)	⬆️

---

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 43c4dcd...96d9671. Read the comment docs.

[jreback]

lgtm.

[gfyoung]

Thanks @datapythonista !