Direct emissions method docs update

[kndehaan]

Context

Documentation about the direct emissions method has been updated according to the recent developments and new implementations of the method.

Checklist

• I have tested these changes
• I have updated documentation as needed
• I have tagged the relevant people for review

[kndehaan]

Update documentation with points below @robindesmit-quintel

1. Update existing sections with Klimaattafels addition:

• "Data export" section: add Klimaattafel
• "Chart" section: add Klimaattafel

2. Add mapping ETM sectors to IPCC categories

3. Add Klimaattafels section. Including short description of what it is. Should only be used for Dutch datasets.