Improve documentation of sits 1.5.2

[gilbertocamara]

There are several improvements required in the documentation of sits-1.5.2, as follows:

1. Improve the explanation of how to build data cubes using local data.
2. Create a separate chapter to explain how to merge data cubes.
3. Revise the uncertainty chapter to include a more accurate description of the method's limitations.
4. Revise the chapter on Bayesian smoothing to align with the paper published in Remote Sensing MDPI.
5. Include a section (chapter on interoperability?) on the methods to export data to Python users.