fix: add Makefile-based local setup commands for the website

## Problem

The README explains how to run the website locally, but the setup is still split across manual commands:

- `git submodule update --init --recursive`
- `npm install`
- `hugo server -D`

This works, but it is a bit verbose for new contributors and easy to miss if someone is just trying to get the site running quickly.

## Proposed fix

Add a root `Makefile` with simple helper targets such as:

- `make setup`
- `make serve`
- `make build`

Then update the README local development instructions to use those commands instead of the longer manual sequence.

## Acceptance criteria

- A root `Makefile` exists with clear local dev targets.
- The README local setup section uses the `make` commands.
- New contributors can start the site with fewer manual steps.
- Existing behavior stays the same.

## Why this helps

This makes the local setup flow easier to follow and reduces friction for first-time contributors.