Improve setup documentation. Add ENV vars for easier configuration

[microstudi]

This PR improves documentation on how to setup this plugin on Drupal/CiViCRM side and allows the use of coherent ENV vars instead of having to create initiatlizers (although compatibility is maintained).

It also adds a default icon to the login page:

[codecov]

Codecov Report

Base: 59.74% // Head: 61.37% // Increases project coverage by +1.63% 🎉

Coverage data is based on head (03e98dc) compared to base (41fcc5a).
Patch coverage: 74.19% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #26      +/-   ##
==========================================
+ Coverage   59.74%   61.37%   +1.63%     
==========================================
  Files          77       77              
  Lines        1463     1468       +5     
==========================================
+ Hits          874      901      +27     
+ Misses        589      567      -22     

Impacted Files	Coverage Δ
lib/decidim/civicrm/version.rb	0.00% <0.00%> (ø)
...rators/decidim/civicrm/templates/civicrm_config.rb	0.00% <0.00%> (ø)
...s/decidim/civicrm/templates/test_civicrm_config.rb	0.00% <0.00%> (ø)
lib/decidim/civicrm/engine.rb	91.83% <75.00%> (-5.99%)	⬇️
...concerns/decidim/civicrm/needs_civicrm_snippets.rb	100.00% <100.00%> (ø)
lib/decidim/civicrm.rb	100.00% <100.00%> (ø)
lib/decidim/civicrm/api.rb	95.83% <100.00%> (ø)
lib/omniauth/strategies/civicrm.rb	96.77% <100.00%> (+38.15%)	⬆️
... and 1 more

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