build: add mdbook tools to nix devShell

Add mdbook, mdbook-mermaid, and mdbook-linkcheck to the nix development
shell for building documentation locally.

These tools are required by the docs build scripts (scripts/docs.sh and
scripts/docs_build.sh).

Author: ammar-agent

flake.nix

@@ -130,6 +130,11 @@
             bash
             nixfmt-rfc-style
 
+            # Documentation
+            mdbook
+            mdbook-mermaid
+            mdbook-linkcheck
+
             # Terminal bench
             uv
             asciinema