Reorganize and improve the documentation

[MLopez-Ibanez]

I reorganized the documentation in commit 9a03330, but it still does not look as nice as other Python packages or the documentation of the R package (https://mlopez-ibanez.github.io/eaf/). In particular:

• Move all developer information and instructions to a different file (similar to https://github.com/poliastro/poliastro/blob/main/CONTRIBUTING.md)
• Include CONTRIBUTING.md in the sphinx documentation (similar to https://docs.poliastro.space/en/stable/contributing.html)
• Rewrite README.md to give brief intro, installation instructions, etc.
• Use README.md as the index of the sphinx documentation.
• The API reference page does not look very nice compared to other packages. See:
  • https://numpy.org/doc/stable/reference/index.html
  • https://docs.poliastro.space/en/stable/api.html
• We should have a Release Notes or Changelog page:
  • https://docs.poliastro.space/en/stable/changelog.html
  • https://numpy.org/doc/stable/release.html
• The separation in submodules in the API reference does not really make sense because all functions are exported from __init__.py so those submodules are invisible to the user. It would be better to document all functions in the same page or separate each function on a different page:
  • https://docs.poliastro.space/en/stable/api.html (separates between high-level and core, we could separate between Handling datasets, Quality metrics, EAF computation, Plotting)
  • https://numpy.org/doc/stable/reference/index.html is even better organized.