Updates to documentation

[orausch]

• Point out that ONNX Runtime is not required
• Fix build errors on RTD
• Fix documentation of python_pure_op_implementation ops

[codecov[bot]]

Codecov Report

Merging #118 (d04f154) into master (3c799c5) will increase coverage by 0.03%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #118      +/-   ##
==========================================
+ Coverage   69.70%   69.73%   +0.03%     
==========================================
  Files          65       65              
  Lines        7232     7239       +7     
==========================================
+ Hits         5041     5048       +7     
  Misses       2191     2191              

Impacted Files	Coverage Δ
daceml/onnx/op_implementations/utils.py	100.00% <100.00%> (ø)

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