Fix pkgcheck status

[GFabien]

Try to conform to pkgcheck:

• Add missing items to documentation,
• Add CONTRIBUTING and CODE_OF_CONDUCT files,
• Add citation (will need to be replaced with JSS citation),
• Add pkgdown configuration,
• Add github actions,
• Change name in DESCRIPTION to match name on CRAN,
• Add codemeta.json,
• Add JSS vignette,
• Update documentation.

[codecov-commenter]

Codecov Report

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