Graduate Python feature server

[felixwang9817]

What this PR does / why we need it: This PR graduates the Python feature server from alpha status by removing the alpha flag check and modifying the docs. It also deletes some useless doc files.

Which issue(s) this PR fixes:

Fixes #

Does this PR introduce a user-facing change?:

NONE

[codecov-commenter]

Codecov Report

Merging #2263 (7327b1c) into master (7c53177) will increase coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2263      +/-   ##
==========================================
+ Coverage   85.42%   85.46%   +0.04%     
==========================================
  Files         112      112              
  Lines        9593     9586       -7     
==========================================
- Hits         8195     8193       -2     
+ Misses       1398     1393       -5     

Flag	Coverage Δ
integrationtests	73.98% <ø> (-0.27%)	⬇️
unittests	58.62% <ø> (+0.02%)	⬆️

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

Impacted Files	Coverage Δ
sdk/python/feast/feature_store.py	91.05% <ø> (+0.59%)	⬆️
sdk/python/feast/flags.py	100.00% <ø> (ø)
sdk/python/feast/flags_helper.py	100.00% <ø> (+4.76%)	⬆️

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[adchia]

Think you missed a couple spots. e.g. feature_store.yaml files that have python_feature_server: true (there's one in the java serving file + the helm charts at least)

[felixwang9817]

@adchia good catch, I think all of them have been removed now

[adchia]

/lgtm

[feast-ci-bot]

[APPROVALNOTIFIER] This PR is APPROVED

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

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