Update roadmap for 0.6
Update roadmap for 0.6

Move
- Tree-sitter integration (highlighting, folds)
- Better file-change detection

to 0.7+

Move 0.5.x and 0.6 to Completed

neovim/neovim.github.io,neovim/neovim.github.io,neovim/neovim.github.io