Adding favicon on user guide pages

[caioguedes]

Fixes #34

The icon file was extracted from the main page of the documentation.

[CLAassistant]

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[sdelamo]

do we have a favicon.ico you are referencing in this PR?
I assume we will have to copy https://micronaut.io/wp-content/uploads/2021/02/favicon.ico in the img folder

[caioguedes]

do we have a favicon.ico you are referencing in this PR? I assume we will have to copy https://micronaut.io/wp-content/uploads/2021/02/favicon.ico in the img folder

Exactly, I extracted the icon from the main page https://micronaut.io/wp-content/uploads/2021/02/favicon.ico, do we should point the icon to this url or the raw url in the git repository?