feat: Convert the blog to mdbook

Replaced the custom HTML/CSS blog with mdbook. Gained a sidebar for
navigating sections, subsections, and pages. Applied a sepia-tone,
monospace theme. Blog sections now populate from SUMMARY.md rather than
manually maintained HTML includes.

book.toml

@@ -0,0 +1,30 @@
+[book]
+title = "Blog"
+description = "Stories, adventures, and reflections from life's journey"
+authors = ["Matthew Binning"]
+language = "en"
+multilingual = false
+src = "content"
+
+[build]
+build-dir = "blog"
+create-missing = false
+
+[output.html]
+no-section-label = true
+git-repository-url = "https://forgejo.binning.net/matthew.binning/www"
+site-url = "/blog/"
+default-theme = "light"
+preferred-dark-theme = "light"
+curly-quotes = true
+mathjax-support = false
+copy-fonts = false
+additional-css = ["theme/sepia.css"]
+
+[output.html.fold]
+enable = true
+level = 0
+
+[output.html.playground]
+editable = false
+copyable = false