0.36.0 - egui 0.35 and small improvements

Full diff at https://github.com/emilk/egui_plot/compare/0.35.0..HEAD

PRs

• Replace impl Into<f32> parameters with just f32 #249 by @emilk
• Fix broken relative README links on crates.io #247 by @Burrdi
• Provide element index with label_formatter #138 by @upsj
• Update egui to 0.35 #250 by @lucasmerlin