Combine headers of all pages before generating TOC

By combining all the headers scattered around all the different
pages, before looping over them to generate the ToC, we can now
safely distribute the contents on multiple smaller files. e.g:

```
10-main.md (define h1)
11-sub1.md (define h2 and multiple h3)
12-sub2.md (define h2)
20-second.md (define h1 and multiple h2 and h3)
21-sub1.md (define h2)
```

layouts/partials/funcs/toc_from_pages.html

@@ -1,32 +1,35 @@
-{{ $toc := slice }}
+{{ $combined := slice }}
 {{ range . }}
-    {{ $previousH1 := dict}}
-    {{ $previousLevel := 0 }}
-    {{ $h2s := slice }}
-
     {{ $headers := findRE "<h\\d.*?>(.|\n)*?</h\\d>" .Content }}
-    {{ range $headers }}
-        {{ $level := int (substr . 2 1) }}
-        {{ if le $level 2 }}
-            {{ $title := . | replaceRE "</?h\\d.*?>" "" | htmlUnescape | safeHTML }}
-            {{ $item := dict "level" $level "title" $title }}
+    {{ $combined = $combined | append $headers }}
+{{ end }}
 
-            {{if eq $level 1 }}
-                {{if ne $previousLevel 0 }}
-                    {{ $tocItem := merge $previousH1 (dict "sub" $h2s)}}
-                    {{ $toc = $toc | append $tocItem }}
-                    {{ $h2s = slice }}
-                {{end}}
-                {{ $previousH1 = $item }}
-            {{else}}
-                {{ $h2s = $h2s | append $item}}
+{{ $toc := slice }}
+{{ $previousH1 := dict }}
+{{ $previousLevel := 0 }}
+{{ $h2s := slice }}
+
+{{ range $combined }}
+    {{ $level := int (substr . 2 1) }}
+    {{ if le $level 2 }}
+        {{ $title := . | replaceRE "</?h\\d.*?>" "" | htmlUnescape | safeHTML }}
+        {{ $item := dict "level" $level "title" $title }}
+
+        {{ if eq $level 1 }}
+            {{ if ne $previousLevel 0 }}
+                {{ $tocItem := merge $previousH1 (dict "sub" $h2s) }}
+                {{ $toc = $toc | append $tocItem }}
+                {{ $h2s = slice }}
             {{end}}
-            {{ $previousLevel = $level }}
+            {{ $previousH1 = $item }}
+        {{ else }}
+            {{ $h2s = $h2s | append $item }}
         {{ end }}
+        {{ $previousLevel = $level }}
     {{ end }}
-    {{if ne $previousLevel 0}}
-        {{ $item := merge $previousH1 (dict "sub" $h2s)}}
-        {{ $toc = $toc | append $item }}
-    {{end}}
+{{ end }}
+{{ if ne $previousLevel 0 }}
+    {{ $item := merge $previousH1 (dict "sub" $h2s) }}
+    {{ $toc = $toc | append $item }}
 {{ end }}
 {{ return $toc }}