add data types docs, fix dtype bug at DF level

[cosmicBboy]

Fixes #1058

This PR adds docs to explain in more detail how data type validation works and interacts with the other parts of the pandera schema. It also fixes a bug in DataFrameSchema(dtype=...) where the type validation would occur if no column schemas as specified.

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.54 🎉

Comparison is base (547aff1) 96.68% compared to head (3e58633) 97.23%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1178      +/-   ##
==========================================
+ Coverage   96.68%   97.23%   +0.54%     
==========================================
  Files          65       65              
  Lines        5042     5067      +25     
==========================================
+ Hits         4875     4927      +52     
+ Misses        167      140      -27     

Impacted Files	Coverage Δ
pandera/api/pandas/container.py	99.25% <ø> (ø)
pandera/backends/pandas/container.py	98.31% <100.00%> (+0.08%)	⬆️
pandera/engines/engine.py	98.18% <100.00%> (+7.09%)	⬆️
pandera/engines/pandas_engine.py	95.44% <100.00%> (+3.70%)	⬆️

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