chore: update tooling (mise, justfile, examples)

Add mise.toml for tool version management, rename Justfile to justfile,
add examples/justfile for running Python examples, and update docs.

mise.toml

@@ -0,0 +1,10 @@
+[tools]
+rust = "1.93"
+python = "3.14"
+uv = "latest"
+just = "latest"
+jj = "latest"
+
+"cargo:espup" = "0.16"
+"cargo:espflash" = "3"              # 4.x has broken nix dependency on current stable Rust
+"cargo:probe-rs-tools" = "0.31"