Added migration guide from 3 to 4 to docs

[piotrpospiech]

• added migrating v3 to v4 page
• updated order of pages in getting started section

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.53%. Comparing base (39ef9b4) to head (60d43a9).

Additional details and impacted files

@@           Coverage Diff           @@
##             docs    #1395   +/-   ##
=======================================
  Coverage   95.53%   95.53%           
=======================================
  Files         175      175           
  Lines        2867     2867           
  Branches      773      773           
=======================================
  Hits         2739     2739           
  Misses         59       59           
  Partials       69       69           

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[Monteth]

We recommend using identical versions of all uniforms packages, so let's also point it out here so they won't use uniforms v4 with uniforms-material v3.

Suggested change

	If you want to continue using these packages, you can use the following commands to download the latest version of the package:
	If you want to continue using these packages, we recommend staying with uniforms v3, as we don't guarantee they will work with the newer versions. You can use the following commands to download the latest version of the package:

[kestarumper]

For me the latest version is referring to 4.x, not the latest 3.10.x.