docs: updated Elevation and Responsive docs

[PKulkoRaccoonGang]

Description

On the alpha version of the Paragon documentation site, on the Elevation and Responsive pages, there is not enough complete information about the innovations that have appeared in Paragon (CSS variables).

• added example with CSS variables (Elevation page)
• added documentation with examples about the presence of CSS Custom Media Breakpoints (Responsive page)

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[openedx-webhooks]

Thanks for the pull request, @PKulkoRaccoonGang!

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[PKulkoRaccoonGang]

On this page I am not sure if it is worth deleting information that relates to SCSS variables. Any thoughts?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.82%. Comparing base (e96baef) to head (86c3bac).

Additional details and impacted files

@@           Coverage Diff           @@
##            alpha    #3335   +/-   ##
=======================================
  Coverage   93.82%   93.82%           
=======================================
  Files         229      229           
  Lines        3836     3836           
  Branches      916      920    +4     
=======================================
  Hits         3599     3599           
  Misses        230      230           
  Partials        7        7           

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