Documentation update

[sbrunato]

• Move API User guide / Overview to Getting Started and rename it to Features overview.
• In API User guide / * pages, add a paragraph that introduces features listed below with links to dedicated sections.
• In Getting started, add a new introduction, and a common thread between subsections. Inspire from https://kubernetes.io/docs/home/ for that.
  • Rename Available providers to Featured providers, add an introduction in this section and also a link to registration page
  • Rename EO Product types to Product types
• Remove Site contents from index
• Replace Getting Started and API User guide sub-sections listing in their index with gallery-grid boxes like on pydata documentation (from which sphinx-book-theme inherits)