updating security documentation: allowed_urls

[agunapal]

Description

Updates security guidelines to configure allowed_urls in production

Fixes #(issue)

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• New feature (non-breaking change which adds functionality)
• This change requires a documentation update

Feature/Issue validation/testing

N?A

Checklist:

• Did you have fun?
• Have you added tests that prove your fix is effective or that this feature works?
• Has code been commented, particularly in hard-to-understand areas?
• Have you made corresponding changes to the documentation?

[codecov]

Codecov Report

Merging #2690 (7a432bb) into master (4d6dbe6) will not change coverage.
The diff coverage is n/a.

❗ Current head 7a432bb differs from pull request most recent head 49ffe39. Consider uploading reports for the commit 49ffe39 to get more accurate results

@@           Coverage Diff           @@
##           master    #2690   +/-   ##
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  Coverage   72.39%   72.39%           
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  Files          85       85           
  Lines        3956     3956           
  Branches       58       58           
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  Hits         2864     2864           
  Misses       1088     1088           
  Partials        4        4           

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[lxning]

please include this info: https://github.com/pytorch/serve/pull/2685/files#diff-17ed18489a956f326ec0fe4040850c5bc9261d4631fb42da4c52891d74a59180