chore: Add documentation for custom offline stores

[kevjumba]

Signed-off-by: Kevin Zhang kzhang@tecton.ai

What this PR does / why we need it:
Add some documentation on adding offline stores.
Which issue(s) this PR fixes:

Fixes #2411

[codecov-commenter]

Codecov Report

Merging #2430 (5528179) into master (e7dd4b7) will decrease coverage by 26.00%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           master    #2430       +/-   ##
===========================================
- Coverage   83.50%   57.49%   -26.01%     
===========================================
  Files         122      123        +1     
  Lines       10714    10762       +48     
===========================================
- Hits         8947     6188     -2759     
- Misses       1767     4574     +2807     

Flag	Coverage Δ
integrationtests	?
unittests	57.49% <ø> (+0.35%)	⬆️

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

Impacted Files	Coverage Δ
...python/feast/infra/offline_stores/offline_store.py	69.04% <ø> (-14.29%)	⬇️
.../integration/online_store/test_online_retrieval.py	17.39% <0.00%> (-82.61%)	⬇️
sdk/python/tests/utils/online_read_write_test.py	20.68% <0.00%> (-79.32%)	⬇️
...fline_store/test_universal_historical_retrieval.py	23.50% <0.00%> (-76.50%)	⬇️
sdk/python/feast/wait.py	23.52% <0.00%> (-70.59%)	⬇️
sdk/python/feast/infra/online_stores/redis.py	27.04% <0.00%> (-69.19%)	⬇️
...gration/registration/test_feature_service_apply.py	31.25% <0.00%> (-68.75%)	⬇️
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%)	⬇️
...dk/python/tests/integration/e2e/test_validation.py	35.18% <0.00%> (-64.82%)	⬇️
... and 65 more

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[felixwang9817]

just a couple of nits

[felixwang9817]

/lgtm

[feast-ci-bot]

[APPROVALNOTIFIER] This PR is APPROVED

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

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