Update curves documentation

[kndehaan]

The docs page about Curves is outdated and can generally use an update. Some points that stand out (but not limited by):

• Many provided sources are NL specific and do not apply to EU datasets
• Some comments mentioned in the Comment column are quite unnecessary (e.g. Update when data becomes available) and can be omitted
• Check for completeness: new profiles might have been added in the mean time but are perhaps not mentioned here in the docs (for instance electric planes and ships).
• Price curves are not mentioned here
• Time curves are mentioned, I'm not familiar with this, do we still use those?