feat(deploy): publish to GitHub Pages and Cloudflare Pages

Build the site from one source and deploy it to both hosts from GitHub
Actions, so neither depends on a host's default Jekyll toolchain.

- Gemfile/Gemfile.lock pin Jekyll 4.4.1 on Ruby 3.4; both workflows use
  them, which keeps the two sites on an identical toolchain
- _config.yml carries the GitHub Pages url and baseurl, fixing asset
  paths that previously resolved off-site; _config.cloudflare.yml layers
  over it for the pages.dev root
- wrangler.jsonc declares the Pages project name and output directory
- 404.html gives both hosts a page for unmatched routes
- exclude README.md from the output and stop shipping CSS sourcemaps

_config.cloudflare.yml

@@ -0,0 +1,6 @@
+# Cloudflare Pages overrides, layered on top of _config.yml:
+#   bundle exec jekyll build --config _config.yml,_config.cloudflare.yml
+#
+# The site sits at the domain root here, unlike the GitHub Pages subpath.
+url: https://penny-pincher-provider.pages.dev
+baseurl: ""