add canonical link tags for all pages (#28)

Add canonical link tags in the base template.

Use .Permalink as the source and strip a trailing .html suffix so
canonical URLs are extensionless (e.g. /architecture).

Prefer extensionless canonical URLs because they are shorter and
cleaner, while the .html variants remain valid and accessible on GitHub
Pages.

Canonical tags tell search engines which URL is authoritative, reducing
duplicate-URL ambiguity for indexing and ranking.

This approach is chosen in favor of meddling with `uglyURLs` to retain
compatibility with already-existing links out there on the intertubes.

Author: dscho

layouts/_default/baseof.html

@@ -4,6 +4,8 @@
     <meta charset="utf-8">
     <meta name="viewport" content="width=device-width, initial-scale=1">
     <title>{{ .Title }}</title>
+    {{- $canonical := .Permalink | replaceRE "\\.html$" "" -}}
+    <link rel="canonical" href="{{ $canonical }}">
     <link rel="icon" href="{{ relURL "favicon.ico" }}">
     {{- $style := resources.Get "sass/main.scss" | resources.ExecuteAsTemplate "main.scss" . | css.Sass }}
     <link rel="stylesheet" href="{{ $style.RelPermalink }}">