Add a Makefile for Ruby/bundle/Jekyll commands that I always forget. (#551)

I can never remember how to build the pages. Perhaps `make` helps?
Certainly, it will positively affect all the carbon I'm wasting by
Googling this: Every. Single. Time.

With no arguments it shows help:
```sh
❯ make

help     Show this help
serve    Start a local server and serve the site
build    Build the site
install  Pull all jekyll dependencies
clean    Remove build files
check    Check ruby (bundle) is installed
```

And `make serve` does what 90% of developers are going to want for
checking local changes.

Inspired by (and a little bit stolen from) @t-young31 and @milanmlft
(who are both very cool).

Author: samcunliffe

CONTRIBUTING.md

@@ -75,11 +75,9 @@ Pages all live in the `docs/pages` sub-directory, and are written in markdown.
 To build the webpage locally (for testing)
 
 1. [Install jekyll]
-2. Run `bundle install` from the `docs/` directory of this repository to
-   install dependencies.
-3. Run `bundle exec jekyll serve` from the root directory of this repository.
-   This should fire up a local web server and tell you its address. By default
-   the server will automatically refresh the HTML pages if any changes are made
+2. Run `make serve` from the `docs/` directory of this repository to
+   install dependencies and start a local web server.
+   The server will automatically refresh the HTML pages if any changes are made
    to the markdown sources.
 
 See the [jekyll docs] for more info.

docs/Makefile

@@ -0,0 +1,24 @@
+SHELL := /bin/bash
+.PHONY: help serve build install clean check
+
+help: ## Show this help
+	@echo
+	@grep -E '^[a-zA-Z_0-9-]+:.*?## .*$$' $(MAKEFILE_LIST) \
+		| awk 'BEGIN {FS = ":.*?## "}; {printf "\033[36m%s\033[0m|%s\n", $$1, $$2}' \
+        | column -t -s '|'
+	@echo
+
+serve: install ## Start a local server and serve the site
+	bundle exec jekyll serve
+
+build: install ## Build the site
+	bundle exec jekyll build
+
+install: check ## Pull all jekyll dependencies
+	bundle install
+
+clean: ## Remove build files
+	rm -rf _site
+
+check: ## Check ruby (bundle) is installed
+	@command -v bundle &> /dev/null || { echo >&2 "Please install ruby >= 2.7 to use Jekyll"; exit 1; }