docs: Update intro documentation page

[adchia]

What this PR does / why we need it:
This updates the intro Feast documentation page to clarify on:

• What Feast is
• What Feast doesn't fully do
• Who Feast is built for
• Example use cases

Which issue(s) this PR fixes:

Fixes #

[codecov-commenter]

Codecov Report

Merging #2982 (4d8b2f3) into master (2680f7b) will decrease coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2982      +/-   ##
==========================================
- Coverage   77.82%   77.78%   -0.05%     
==========================================
  Files         192      192              
  Lines       16427    16427              
==========================================
- Hits        12784    12777       -7     
- Misses       3643     3650       +7     

Flag	Coverage Δ
integrationtests	67.87% <ø> (-0.15%)	⬇️
unittests	59.06% <ø> (ø)

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

Impacted Files	Coverage Δ
sdk/python/tests/utils/basic_read_write_test.py	93.54% <0.00%> (-6.46%)	⬇️
.../integration/online_store/test_online_retrieval.py	96.84% <0.00%> (-3.16%)	⬇️
sdk/python/feast/infra/online_stores/datastore.py	87.96% <0.00%> (-1.39%)	⬇️
...east/infra/materialization/lambda/lambda_engine.py	93.75% <0.00%> (+1.04%)	⬆️

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[achals]

/lgtm

[feast-ci-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: achals, adchia, kevjumba

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• OWNERS [achals,adchia,kevjumba]

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[kevjumba]

/lgtm