Add data source implementations to RTD docs

[felixwang9817]

Signed-off-by: Felix Wang wangfelix98@gmail.com

What this PR does / why we need it: This PR adds BigQuerySource, RedshiftSource, and FileSource to Feast's RTD docs.

Which issue(s) this PR fixes:

Fixes #

Does this PR introduce a user-facing change?:

NONE

[achals]

/lgtm

[feast-ci-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: achals, felixwang9817

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[codecov-commenter]

Codecov Report

Merging #2097 (ef76d54) into master (54d0f3a) will decrease coverage by 25.34%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           master    #2097       +/-   ##
===========================================
- Coverage   83.44%   58.10%   -25.35%     
===========================================
  Files         100      100               
  Lines        8077     8077               
===========================================
- Hits         6740     4693     -2047     
- Misses       1337     3384     +2047     

Flag	Coverage Δ
integrationtests	?
unittests	58.10% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
.../integration/online_store/test_universal_online.py	15.34% <0.00%> (-82.80%)	⬇️
.../integration/online_store/test_online_retrieval.py	17.39% <0.00%> (-82.61%)	⬇️
...fline_store/test_universal_historical_retrieval.py	19.29% <0.00%> (-80.12%)	⬇️
sdk/python/tests/utils/online_read_write_test.py	20.68% <0.00%> (-79.32%)	⬇️
...gration/registration/test_feature_service_apply.py	31.25% <0.00%> (-68.75%)	⬇️
sdk/python/feast/infra/online_stores/redis.py	26.97% <0.00%> (-67.11%)	⬇️
sdk/python/tests/data/data_creator.py	34.78% <0.00%> (-65.22%)	⬇️
sdk/python/tests/integration/e2e/test_usage_e2e.py	35.00% <0.00%> (-65.00%)	⬇️
...s/integration/registration/test_universal_types.py	36.73% <0.00%> (-63.27%)	⬇️
...istration/test_universal_odfv_feature_inference.py	38.29% <0.00%> (-61.71%)	⬇️
... and 49 more

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