Intro page for rgbif

[CecSve]

Include:

• [x] short introduction
• [x] link to technical documentation
• [x] 'getting started with rgbif' - a small tutorial showing how the package can be used

[MattBlissett]

https://tech-docs.gbif-dev.org/en/data-use/rgbif

I copied over an existing page just as a starter.