Update the CSFML & SFML.Net page

[eXpl0it3r]

• Update website for SFML.Net 2.3
• Update website for SFML.Net 2.4
• Update website for CSFML 2.5.1
• Update website for CSFML 2.5.2
  • Build CSFML 2.5.2 for macOS & update website
• Update website for SFML.Net 2.5
• Update website for CSFML 2.6.0
  • Build CSFML 2.6.0 for macOS & update website
• Update website for SFML.Net 2.6
  • Finalize SFML.Net 2.6
• Provide link to NuGet package
• Rewrite the text above on the download page to reflect the current state of SFML.Net and .Net Standard
• Explain the NuGet setup