[Documentation]Requires unstable Neovim on Ubuntu...

[mgajda]

When running lua require('gemini').setup() on stock Ubuntu 25.04, it reports that lua version is too old.
Workaround is to install unstable Neovim from (from repository sudo add-apt-repository ppa:neovim-ppa/unstable).

But since the error is silent on startup, it would be nice to add it to documentation.