Restructure as a standard chart repository

[LeoColomb]

This pull request aims to give to this git repository a typical/standard structure for a Helm chart repository.

/ 				<- Git root
	charts/ 	<- Helm chart repo roots
		netbox/ <- Netbox chart
		?/		<- Other can come in future
	README		<- Git repo meta-files 
	...		<- Other git meta-files 

Note that none of the files is altered, only their location.
Single exception, the .gitignore to reflect the change of paths.

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Direct follow up of #162 (comment)