Add examples for how to use environment variables

[BogdanIrimie]

Add docs example on how to override topaz config using environment variables.

[github-actions]

Pull Request Test Coverage Report for Build 6247629947

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 30.016%

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Totals
Change from base Build 6245314775:	0.0%
Covered Lines:	1119
Relevant Lines:	3728

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💛 - Coveralls

[gertd]

I assume token_placeholder and token_placeholder are environment variables? If so can we please make the UPPERCASE, so they are easier to spot when reading a config file.

[BogdanIrimie]

I assume token_placeholder and token_placeholder are environment variables? If so can we please make the UPPERCASE, so they are easier to spot when reading a config file.

In this case, they are just random strings. The environment variable name determines the yaml path where the substitution will happen. I tried to improve the docs to describe the 2 cases (viper env substitution and interpolation).