docs(Layouts): fix vertical alignment on LoginPage

[ashvini-maurya]

The vertical alignment of login form is fixed.
Earlier the grid height was given 100% and now I changed grid height to 100vh. Now the login form will be always centered irresective of screen height.
This PR fixes this issue

Fixes #3620.

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Here is a list of things that will help get it across the finish line:

• Run yarn lint locally to catch formatting errors. This will fix some errors automatically, commit and push any changes.
• Run yarn test locally to catch errors. This ensures all components still behave as they should.
• Run yarn start to run the doc site locally and try a few pages, ensuring everything is in good working order.
• Include tests when adding/changing behavior.

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[codecov-io]

Codecov Report

Merging #3623 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3623   +/-   ##
=======================================
  Coverage   99.81%   99.81%           
=======================================
  Files         174      174           
  Lines        2744     2744           
=======================================
  Hits         2739     2739           
  Misses          5        5

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