add rule id to docs

[jrobison-sb]

• I noticed a deprecation warning of Deprecation warning : Use 'id' (AC_AWS_0058) instead of 'reference_id' (AWS.RDS.DataSecurity.High.0414) and wanted to fix it in my config file, but then noticed that those ID's (AC_AWS_0058) aren't documented anywhere other than within the json of each rule. So I figured I would propose adding those ID's to the docs.
• I now see that maybe this repo isn't even the source of truth for the docs anymore? But the https://github.com/accurics/runterrascan.io repo doesn't seem to have any automation to rebuild these, or does it? Anyway, I'm happy to add this if possible but would appreciate guidance if I'm doing it in the wrong place.
• And this also fixes a bug in docs/policies/document_policies.py where it crashed when trying to iterate over files in pkg/policies/opa/rego when it was expecting to iterate over directories.

[sonarqubecloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
1 Code Smell

No Coverage information
0.0% Duplication

[codecov]

Codecov Report

Merging #1136 (7685329) into master (14a634b) will not change coverage.
The diff coverage is n/a.

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##           master    #1136   +/-   ##
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  Coverage   79.09%   79.09%           
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  Files         248      248           
  Lines        6907     6907           
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  Hits         5463     5463           
  Misses       1113     1113           
  Partials      331      331           

[cesar-rodriguez]

Thanks @jrobison-sb !

@sangam14 we need an easier/better way to sync these with runterrascan.io