docs: Functionality matrix for online stores

[felixwang9817]

What this PR does / why we need it:

Which issue(s) this PR fixes:

Fixes #

[codecov-commenter]

Codecov Report

Base: 66.85% // Head: 58.36% // Decreases project coverage by -8.48% ⚠️

Coverage data is based on head (1f1cc6b) compared to base (d7b0c52).
Patch has no changes to coverable lines.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3007      +/-   ##
==========================================
- Coverage   66.85%   58.36%   -8.49%     
==========================================
  Files         175      209      +34     
  Lines       15848    17617    +1769     
==========================================
- Hits        10595    10283     -312     
- Misses       5253     7334    +2081     

Flag	Coverage Δ
integrationtests	?
unittests	58.36% <ø> (?)

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

Impacted Files	Coverage Δ
...sts/integration/registration/test_universal_cli.py	20.20% <0.00%> (-79.80%)	⬇️
...ts/integration/offline_store/test_offline_write.py	26.08% <0.00%> (-73.92%)	⬇️
...fline_store/test_universal_historical_retrieval.py	28.75% <0.00%> (-71.25%)	⬇️
...ests/integration/e2e/test_python_feature_server.py	29.50% <0.00%> (-70.50%)	⬇️
...dk/python/tests/integration/e2e/test_validation.py	27.55% <0.00%> (-69.30%)	⬇️
...s/integration/registration/test_universal_types.py	32.25% <0.00%> (-67.75%)	⬇️
sdk/python/feast/infra/online_stores/redis.py	28.39% <0.00%> (-66.58%)	⬇️
sdk/python/tests/integration/e2e/test_usage_e2e.py	33.87% <0.00%> (-66.13%)	⬇️
sdk/python/tests/data/data_creator.py	34.78% <0.00%> (-65.22%)	⬇️
...n/tests/integration/registration/test_inference.py	35.71% <0.00%> (-64.29%)	⬇️
... and 163 more

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[adchia]

/lgtm

[feast-ci-bot]

[APPROVALNOTIFIER] This PR is APPROVED

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

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