DOC: Use google-style docstrings in _page.py

[MartinThoma]

This PR is a step towards Google-style docstrings.

• I think they are way easier to read within the source code
• We don't have the type annotation + the type in the docstring. This is a pro (no duplication) and a con (less control over how it's shown in the docs)

Deprecated methods were not touched

[MartinThoma]

This is what I mean with the readability:

[codecov]

Codecov Report

Base: 92.03% // Head: 92.57% // Increases project coverage by +0.54% 🎉

Coverage data is based on head (17632b0) compared to base (9b23a39).
Patch has no changes to coverable lines.

❗ Current head 17632b0 differs from pull request most recent head e089e80. Consider uploading reports for the commit e089e80 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1503      +/-   ##
==========================================
+ Coverage   92.03%   92.57%   +0.54%     
==========================================
  Files          32       32              
  Lines        5976     5970       -6     
  Branches     1163     1163              
==========================================
+ Hits         5500     5527      +27     
+ Misses        312      284      -28     
+ Partials      164      159       -5     

Impacted Files	Coverage Δ
PyPDF2/_page.py	91.96% <ø> (+2.39%)	⬆️
PyPDF2/xmp.py	92.10% <0.00%> (ø)
PyPDF2/types.py	100.00% <0.00%> (ø)
PyPDF2/errors.py	100.00% <0.00%> (ø)
PyPDF2/_merger.py	93.18% <0.00%> (ø)
PyPDF2/_reader.py	90.32% <0.00%> (ø)
PyPDF2/generic/_data_structures.py	89.97% <0.00%> (ø)
PyPDF2/_writer.py	85.11% <0.00%> (+1.16%)	⬆️
PyPDF2/_utils.py	99.47% <0.00%> (+2.01%)	⬆️
... and 1 more

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