Update how-to-start guide

[64J0]

Description:

With this PR, I'm:

• Updating the how-to-start guide due to the major .NET update of the project (issue link).
• Update the command used to start the sample application in watch mode. It was required to change due to the lack of FAKE in the template.

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This PR is related to this issue, where a user had difficulty to start using this tool:

• How To Start a Saturn Project #349

It is also linked to this other PR, where I sent some code to update the Saturn template and remove some broken configurations (they are broken in .NET 6, previously they were required):

• Fix the code generation feature due to .NET 6 Saturn.Cli#39

[cartermp]

Thanks!