[Docs] Add sensitive fields reference to platform configuration

[TomerShor]

Document how to enable and set up sensitive fields masking in the platform configuration.

[liranbg]

Suggested change

	Sensitive fields are fields that contain sensitive information such as passwords, tokens, etc.
	When masking is enabled, these fields are encrypted in a secret, and are decrypted when the function is deployed.
	There are some config fields that are [masked by default](https://github.com/nuclio/nuclio/blob/development/pkg/platformconfig/types.go#L303-L340), and you can add custom sensitive fields to mask by specifying the regex to the path in the function configuration.
	The masked fields are replaced with references (`$ref`) in the function configuration.
	Example:
	Nuclio resources (currently function configuration only) may contain sensitive information such as passwords, tokens, etc.
	When masking sensitive fields feature is enabled, these fields gets obfuscated, their raw values stored aside (Kubernetes secret), and populated internally, for when it is needed (i.e function deployment)
	There are some config fields that are [masked by default](https://github.com/nuclio/nuclio/blob/development/pkg/platformconfig/types.go#L303-L340), and you can add custom sensitive fields to mask by specifying the regex to the path in the function configuration.
	The masked fields are replaced with references (`$ref`) in the function configuration.
	Example:

While writing this doc, I think it is clear we need to extend the capability for more nuclio resources (project, api gateway, etc)