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)

Issue: #3171

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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 93.91%. Comparing base (69e4fb1) to head (d6ce36a).

Additional details and impacted files

@@              Coverage Diff              @@
##           release-23.x    #3335   +/-   ##
=============================================
  Coverage         93.91%   93.91%           
=============================================
  Files               232      232           
  Lines              3943     3943           
  Branches            933      933           
=============================================
  Hits               3703     3703           
  Misses              236      236           
  Partials              4        4           

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[brian-smith-tcril]

LGTM!