update variables docs

[andreafalzetti]

Related to: https://okteto.atlassian.net/browse/DEV-282

I am updating the docs related to Okteto Variables to highlight certain behaviours (such as order of priority) but also to make it much easier for someone to figure out what type of variables exist and how to use them, providing use cases and examples.

Additionally, I moved some stuff around, feel free to provide your feedback on the following:

• Moved "Feature Flags" out of "Variables" because the 2 are just slightly related and Feature Flags is hidden inside it now.
• Created an explicit page Core -> Credentials -> Environment Variables instead of using the Okteto CLI -> context section to explain authentication.

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[pchico83]

@andreafalzetti related to these changes:

Additionally, I moved some stuff around, feel free to provide your feedback on the following:

Moved "Feature Flags" out of "Variables" because the 2 are just slightly related and Feature Flags is hidden inside it now.
Created an explicit page Core -> Credentials -> Environment Variables instead of using the Okteto CLI -> context section to explain authentication.

Both look great to me!

[andreafalzetti]

Thanks @pchico83 for the help on fixing the links. Merging now 🚀