Make the repository public

[ranocha]

Here are some things we should keep in mind when making the repo public:

• [x] Make the repo public
• [x] Add some badges to the README.md file (#5)
• [x] Integrate with Coverage report repos and activate the corresponding part in .github/workflows/CI.yml (#8)
• [x] Set up Documenter.jl (including the GItHub secret etc.) (#6)
• [x] Register it in the long run (https://github.com/JuliaRegistries/General/pull/103552)
• [ ] Update installation instructions in the README.md and the docs (#53)