Update custom curves documentation

[kndehaan]

Related to this PR that ensures that uploaded custom curves with capacity_profile are also downloaded again in the same format, there is a need to update documentation accordingly.

This PR therefore updates documentation to:

• Improve clarity about in which format custom curves should be uploaded
• Indicate that the downloaded custom curves are obtained in the same format
• Underline that these curves represent scenario input and are thus not the outcome of scenario calculations

Along the way, some outdated documentation regarding custom curves is deleted or updated where relevant.

Goes with:

• Rescaling curves etengine#1565
• Update custom curves documentation #230
• Update custom curves documentation etmodel#4494