Fix README links and update structure

[peanutfun]

Changes proposed in this PR:

• Fix links, have them point to new docs
• Update structure:
  • Make data API less prominent
  • Fewer "click here" links
  • Separate introduction from overview

NOTE: Merging into develop is okay. Broken links in main are fixed by redirects on ReadTheDocs.

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

[peanutfun]

@chahank could you please have a look?

[peanutfun]

Based on feedback from the CLIMADA Days:

Suggested change

	A [publication repository](https://github.com/CLIMADA-project/climada_papers) contains scientific, peer-reviewed articles that explain software components implemented in CLIMADA.
	A [publication repository](https://github.com/CLIMADA-project/climada_papers) contains code and data used in CLIMADA case-studies for scientific, peer-reviewed articles.

[chahank]

hmmm I am not sure what feedback this is referring too. Maybe the one saying that it is hard to find examples of CLIMADA applications? In this case I would formulate it differently. Maybe?

Suggested change

	A [publication repository](https://github.com/CLIMADA-project/climada_papers) contains scientific, peer-reviewed articles that explain software components implemented in CLIMADA.
	A [repository](https://github.com/CLIMADA-project/climada_papers) contains case-study examples most of them in the form of scientific, peer-reviewed articles that further explain software components implemented in CLIMADA.

Otherwise I feel it is even more innaccessible to practitionners that are looking for examples, not scientific studies per se.

[peanutfun]

NOTE: Merging into develop is okay. Broken links in main are fixed by redirects on ReadTheDocs.

Thinking about this, it might make sense to merge into main because otherwise these changes will not "manifest" until the next release.

[chahank]

Suggested change

1. [Core (`climada_python`)](https://github.com/CLIMADA-project/climada_python) contains all the modules necessary for computing probabilistic impact, averted damage, and uncertainty, sensitivity analysis and impact function calibration. Data for hazard, exposures and impact functions can be obtained from the [data API](https://github.com/CLIMADA-project/climada_python/blob/main/doc/user-guide/climada_util_api_client.ipynb). [Litpop](https://github.com/CLIMADA-project/climada_python/blob/main/doc/user-guide/climada_entity_LitPop.ipynb) is included as demo Exposures module, and [Tropical cyclones](https://github.com/CLIMADA-project/climada_python/blob/main/doc/user-guide/climada_hazard_TropCyclone.ipynb) is included as a demo Hazard module.

1. [Core (`climada_python`)](https://github.com/CLIMADA-project/climada_python) contains all the modules necessary for computing probabilistic impact, averted damage, and peform uncertainty, sensitivity analysis and impact function calibration. Demonstration data for hazard, exposures and impact functions can be obtained from the [data API](https://github.com/CLIMADA-project/climada_python/blob/main/doc/user-guide/climada_util_api_client.ipynb). [Litpop](https://github.com/CLIMADA-project/climada_python/blob/main/doc/user-guide/climada_entity_LitPop.ipynb) is included as demo Exposures module, and [Tropical cyclones](https://github.com/CLIMADA-project/climada_python/blob/main/doc/user-guide/climada_hazard_TropCyclone.ipynb) is included as a demo Hazard module.