Add developper docs for project-folder

[timotheeguerin]

fix #3695

[azure-pipelines]

You may test this build by running autorest --reset and then either:

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Add the following CLI flags

Pacakge	Flag	Description
@autorest/core	--version:https://tinyurl.com/yzjzvzem	For changes to autorest core.
@autorest/modelerfour	--use:https://tinyurl.com/yz5s8by5	For changes to modelerfour.

Or with all

autorest --version:https://tinyurl.com/yzjzvzem --use:https://tinyurl.com/yz5s8by5

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or use the following in your autorest configuration:

# For changes to autorest core
version: "https://tinyurl.com/yzjzvzem"

# For changes to modelerfour
use-extension:
  "@autorest/modelerfour": "https://tinyurl.com/yz5s8by5"

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If this build is good for you, give this comment a thumbs up. (👍) And you should run `autorest --reset` again once you're finished testing to remove it.