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.

---

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

• https://github.com/SaturnFramework/Saturn/issues/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):

• https://github.com/SaturnFramework/Saturn.Cli/pull/39