Replace global Zenodo DOI on badges and citation

[santisoler]

Replace the reserved Zenodo DOI for the first release with the global
Zenodo DOI for Harmonica. Zenodo makes available the global DOI of the
release after it's released. So, the only DOI available at the time was
the one corresponding to the first release: v0.1.0. It's better to have
the global DOI on the badge from README.rst and on citation
instructions on CITATION.rst.

Reminders

• Run make format and make check to make sure the code follows the style guide.
• Add tests for new features or tests that would have caught the bug that you're fixing.
• Add new public functions/methods/classes to doc/api/index.rst.
• Write detailed docstrings for all functions/methods.
• If adding new functionality, add an example to the docstring, gallery, and/or tutorials.

[leouieda]

👍 thanks! Merge away when ready.