Remove broken badges from README, and tidy content

[tompollard]

This pull request makes some minor improvements to the README.md (https://github.com/MIT-LCP/wfdb-python/blob/main/README.md). The changes are:

1. Remove the broken "Tests Workflow" badge (not helpful in my opinion, and we can add back later if we decide it is important).
2. Remove the broken "Supported Python Versions" button (not necessary in my opinion, and we can add back later if we decide it is important).
3. Updates the DOI badge so that it points to the latest version of the package (which is generally what people will want).
4. Makes minor updates to the content for readability (for example, adding an introduction to WFDB and moving development-related content to the development section).

[Chrystinne]

Looks good, @tompollard!