Add documentation and website

[statnmap]

Thank you for your package.
To allow a broader use, I suggest to make sure the checks are ok, and a minimum documentation is available.

You will find modifications in this PR:

• All checks are good
• A readme.Rmd with explanation on how to set up the API
• A readme.Rmd with a first example
• An update of dependencies in DESCRIPTION using attachment::att_amend_desc() to allow installation of the package with its dependencies
• A set up of GitHub Actions to check automatically each commit
• A set up of GitHub Actions to serve a pkgdown website (as demanded in Any plans to create a pakgdown documentation site? #16)
  • To make it available after you accept the PR, you will have to go in : Settings > Pages > Deploy from a branch : gh-pages : Root > Save
  • You will then see the website there I guess : https://Eflores89.github.io/notionR/
• A "dev/" directory with the main functions used to set all these steps

Do not hesitate to ask me questions about theses changes if you want.