Create cuGraph developers guide

[ChuckHastings]

We have long needed a developer's guide. This PR creates one. The expectation is that this document will be adapted over time.

I have copied the version from cuDF, edited things to make it cuGraph focused. There are documentation holes here that will need to eventually be filled in. However, it's probably useful to get something with holes in it that is approved so we can move forward with some documented standard.

[BradReesWork]

rerun tests

[codecov-io]

Codecov Report

Merging #1431 (69e64b6) into branch-0.19 (89bffa5) will decrease coverage by 0.60%.
The diff coverage is n/a.

@@               Coverage Diff               @@
##           branch-0.19    #1431      +/-   ##
===============================================
- Coverage        60.72%   60.12%   -0.61%     
===============================================
  Files               70       70              
  Lines             3132     3132              
===============================================
- Hits              1902     1883      -19     
- Misses            1230     1249      +19     

Impacted Files	Coverage Δ
python/cugraph/community/leiden.py	35.71% <0.00%> (-57.15%)	⬇️
python/cugraph/community/ecg.py	54.54% <0.00%> (-45.46%)	⬇️
python/cugraph/community/louvain.py	57.14% <0.00%> (-35.72%)	⬇️
python/cugraph/utilities/utils.py	70.86% <0.00%> (-0.79%)	⬇️

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[BradReesWork]

@gpucibot merge