[Documentation] Add configuration file description

[clbartoli]

The node configuration file, usually called witnet.toml, needs to be described in a small doc that will be linked in the toml.

• Create a folder called configuration
• In that folder create the file called toml-file with the configuration file description

[aesedepece]

A good starting point is the Rust implementation itself. The definition of the fields may already contain documentation.

[Tommytrg]

As suggested, we have used the definition of the fields in the codebase to generate the page

https://docs.witnet.io/node-operators/advanced-setups/configuration-file