feat(site): add section and heading anchors for deep-linking

Landing sections now carry ids (status, features, faq, collab, …) so any
section is directly linkable (e.g. /#faq). About and Legal headings get a
discreet, hover-revealed permalink via a goldmark heading render hook.
A scroll-margin-top offset keeps anchored targets clear of the sticky header.

site/layouts/index.html

@@ -1,6 +1,6 @@
 {{ define "main" }}
 {{ $lang := .Site.Language.Lang }}
-<section class="hero">
+<section class="hero" id="top">
   <div class="hero-copy">
     <h1>{{ .Params.hero.title }}</h1>
     <p class="tagline">{{ .Params.hero.tagline }}</p>
@@ -19,7 +19,7 @@
   {{ end }}
 </section>
 
-<section class="status">
+<section class="status" id="status">
   <span class="badge-beta">{{ T "statusBadge" }}</span>
   <div>
     <h2>{{ T "statusTitle" }}</h2>
@@ -27,7 +27,7 @@
   </div>
 </section>
 
-<section class="pillars">
+<section class="pillars" id="pillars">
   {{ range .Params.pillars }}
   <article class="pillar">
     <div class="pillar-icon" aria-hidden="true">{{ .icon }}</div>
@@ -38,7 +38,7 @@
 </section>
 
 {{ with .Params.features }}
-<section class="features">
+<section class="features" id="features">
   <h2>{{ .title }}</h2>
   {{ with .intro }}<p class="features-intro">{{ . }}</p>{{ end }}
   <div class="feature-groups">
@@ -56,7 +56,7 @@
 {{ end }}
 
 {{ $screens := slice "home" "inventory" "market" "calendar" "detail" }}
-<section class="shots">
+<section class="shots" id="screenshots">
   <h2>{{ T "screenshotsTitle" }}</h2>
   <div class="shot-row">
     {{ range $s := $screens }}
@@ -81,14 +81,14 @@
   {{ end }}
 </section>
 
-<section class="values">
+<section class="values" id="values">
   <h2>{{ .Params.values.title }}</h2>
   <ul>
     {{ range .Params.values.points }}<li>{{ . }}</li>{{ end }}
   </ul>
 </section>
 
-<section class="faq">
+<section class="faq" id="faq">
   <h2>{{ T "faqTitle" }}</h2>
   <dl>
     <dt>{{ T "faqFreeQ" }}</dt><dd>{{ T "faqFreeA" }}</dd>
@@ -99,7 +99,7 @@
   </dl>
 </section>
 
-<section class="collab">
+<section class="collab" id="collab">
   <h2>{{ T "collabTitle" }}</h2>
   <p class="lead">{{ T "collabIntro" }}</p>
   <div class="collab-grid">