New resources and webhooks

What's Changed

• Add Peppol receiver field by @stephdewit in #18
• List eInvoicing suppliers by @stephdewit in #20
• Fix retry delay by @stephdewit in #21
• List Peppol Registrations by @stephdewit in #22
• Improve certificates documentation by @stephdewit in #23
• New eInvoicing webhook by @stephdewit in #19

Full Changelog: 0.6.0...0.6.1