Add rule for .md to angular webpack.config.prod.js

[igor-dv]

Issue: #2883 (comment)

What I did

Added a missing rule for .md to the angular prod config

[codecov]

Codecov Report

Merging #2924 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2924   +/-   ##
=======================================
  Coverage   35.99%   35.99%           
=======================================
  Files         429      429           
  Lines        9503     9503           
  Branches      976      978    +2     
=======================================
  Hits         3421     3421           
- Misses       5423     5463   +40     
+ Partials      659      619   -40

Impacted Files	Coverage Δ
...p/angular/src/server/config/webpack.config.prod.js	0% <ø> (ø)	⬆️
app/angular/src/server/config/webpack.config.js	0% <ø> (ø)	⬆️
app/polymer/src/server/config/babel.js	0% <0%> (-100%)	⬇️
app/polymer/src/server/babel_config.js	0% <0%> (-76.67%)	⬇️
app/polymer/src/server/utils.js	0% <0%> (-53.58%)	⬇️
app/react/src/demo/Welcome.js	64.7% <0%> (ø)	⬆️
addons/jest/src/components/Indicator.js	0% <0%> (ø)	⬆️
lib/ui/src/modules/ui/configs/init_panels.js	25% <0%> (ø)	⬆️
addons/storyshots/src/rn/loader.js	53.33% <0%> (ø)	⬆️
...i/src/modules/ui/components/stories_panel/index.js	20.96% <0%> (ø)	⬆️
... and 59 more

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