Restructuring of the documentation

[spjuhel]

This PR replaces PR #972 which was hastily merged and then reverted.
This PR fixes #969

Changes proposed in this PR:

This PR fixes #

Reminders:

• include in guide TOC the two new tutorials climada_entity_Exposures_osm.ipynb and climada_util_local_exceedance_values.ipynb which not yet part of this branch (but are on develop).

PR Author Checklist

• Read the Contribution Guide
• Correct target branch selected (if unsure, select develop)
• Descriptive pull request title added
• Source branch up-to-date with target branch
• Documentation updated
• Tests updated
• Tests passing
• No new linter issues
• Changelog updated

PR Reviewer Checklist

• Read the Contribution Guide
• CLIMADA Reviewer Checklist passed
• Tests passing
• No new linter issues

[spjuhel]

Ok! This is now almost ready for review!

I think I saw some tiny things that remain to be done in the index of Getting started index page, like an image to be added (@NicolasColombi)

Otherwise, everything else is ready to be reviewed !

[NicolasColombi]

Ok! This is now almost ready for review!

I think I saw some tiny things that remain to be done in the index of Getting started index page, like an image to be added (@NicolasColombi)

Otherwise, everything else is ready to be reviewed !

Getting started is now finish an ready for review!

[chahank]

General question: some of the tutorials "show" how to read and write files. So far, there was a folder named results in the tutorial folder where the written files were stored. Should we continue with this practice? Or shall we remove the file writing in the tutorial?