Changed pylibcugraph connected_components APIs to use duck typing for CAI inputs, added doc placeholders

[rlratzel]

• Changed API to use duck-typing on __cuda_array_interface__ types instead of requiring users to access and pass the __cuda_array_interface__ object in directly.
• Added initial placeholder for pylibcugraph docs and docstrings to cython file
• Updated tests to not access the __cuda_array_interface__ attr directly, added fixtures for easier code reuse, added test for invalid inputs, added FIXMEs to track tech debt in how return values are checked.

Tested API changes by running new unit tests, and tested docs by running ./build.sh docs and inspecting the html locally in a browser.

NOTE: since this package is still limited to nightly releases (ie. it has not been part of a release yet), it is using the "non-breaking" label even though the user API has changed.

attn: @BradReesWork and @raydouglass for the pylibcugraph placeholder docs in this PR

[codecov-commenter]

Codecov Report

Merging #1810 (4b825aa) into branch-21.10 (bf64c2c) will increase coverage by 9.85%.
The diff coverage is n/a.

❗ Current head 4b825aa differs from pull request most recent head 0a78d6b. Consider uploading reports for the commit 0a78d6b to get more accurate results

@@               Coverage Diff                @@
##           branch-21.10    #1810      +/-   ##
================================================
+ Coverage         59.85%   69.70%   +9.85%     
================================================
  Files                77      139      +62     
  Lines              3547     8626    +5079     
================================================
+ Hits               2123     6013    +3890     
- Misses             1424     2613    +1189     

Impacted Files	Coverage Δ
python/cugraph/cugraph/__init__.py	100.00% <ø> (ø)
python/cugraph/cugraph/_version.py	44.80% <ø> (ø)
python/cugraph/cugraph/centrality/__init__.py	100.00% <ø> (ø)
...graph/cugraph/centrality/betweenness_centrality.py	90.00% <ø> (ø)
...thon/cugraph/cugraph/centrality/katz_centrality.py	89.47% <ø> (ø)
python/cugraph/cugraph/comms/comms.py	34.52% <ø> (ø)
python/cugraph/cugraph/community/__init__.py	70.37% <ø> (ø)
python/cugraph/cugraph/community/ecg.py	100.00% <ø> (ø)
python/cugraph/cugraph/community/egonet.py	92.30% <ø> (ø)
...ython/cugraph/cugraph/community/ktruss_subgraph.py	84.37% <ø> (ø)
... and 195 more

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[BradReesWork]

@gpucibot merge