docs: Modify the doc following the v0.5 release, part 1

[sylvaincom]

Taking into account comments from @tuscland, @MarieS-WiMLDS and @sylvaincom (see #1008 for the second iteration):

• README
  • update the cross-validation example
• overall
  • the logo on the top left is too big
  • change Twitter into X
  • add Bluesky
  • add LinkedIn
  • add YouTube
  • "Probabl website" -> "Probabl"
• landing page
  • the logo is too small
  • remove the parts about starring
  • remove the badges
  • the orange color is not the correct one
  • remove the right side bar
• install
  • change the tab appearance
  • "Skore" with uppercase if at the beginning of a sentence
  • remove the what's next
• user guide
  • "examples" -> "user guide"
  • "Quick start": to be created
  • "Getting started with skore" -> "Skore product tour"
  • "Tracking items using their history" -> "Tracking items"
  • "Overview of the skore project" -> "Working with projects"
  • "Enhancing cross-validation" -> "Cross-validation"
  • "Enhancing train-test split" -> "Train-test split""
  • remove the "Track" section
  • in the tracking example, rename item_history into passed_item
  • rename "Diagnose" into "Model evaluation"
• contributing
  • Change the first sentence into "Thank you for your interest in contributing to Skore! We welcome contributions from everyone and appreciate you taking the time to get involved."