Add docs for additional install methods (#1059)

doc/installation_nix.md

@@ -0,0 +1,38 @@
+@page installation_nix Nix
+
+> [!note]
+> FTXUI author is not very knowledgeable about Nix. This page has been mostly
+> generated by AI. If you have any suggestions to improve it, please open a
+> PR.
+
+## Nix Flake
+
+FTXUI ships with a `flake.nix` providing both packages and a development shell.
+
+### Build the Library
+
+```bash
+nix build github:ArthurSonzogni/FTXUI
+```
+
+The resulting package is accessible via the `result` link.
+
+### Use as a Dependency
+
+Add FTXUI to your flake inputs:
+
+```nix
+{
+  inputs.ftxui.url = "github:ArthurSonzogni/FTXUI";
+}
+```
+
+Then reference `ftxui.packages.<system>.ftxui` in your outputs.
+
+<div class="section_buttons">
+
+| Previous          |
+|:------------------|
+| [Getting Started](getting-started.html) |
+
+</div>