update the introduction example to use the demo datafiles + documentation update

[vergauwenthomas]

• [x] Update the Quality control example to use demo data
• [x] Convert the gap filling demo to a short example script using the demo data
• [x] Convert the analysis demo to a short example script using the demo data

[vergauwenthomas]

Convert the examples to notebook and render with nbsphinx (see #388)

[vergauwenthomas]

When merging, check if the live documentation is updated.