Rename header template to navbar

To avoid the confusing similarity between _head and _header.

docs/templates/default.liquid

@@ -3,7 +3,7 @@
 {% include "shared/head" %}
 
   <body id="{{slug}}">
-{% include "shared/header" %}
+{% include "shared/navbar" %}
 
     <div class="container">
       <div class="row">

docs/templates/index.liquid

@@ -3,7 +3,7 @@
 {% include "shared/head" %}
 
   <body id="{{slug}}">
-{% include "shared/header" %}
+{% include "shared/navbar" %}
 
     <div class="container">
       <div class="row">

docs/templates/manual.liquid

@@ -3,7 +3,7 @@
 {% include "shared/head" %}
 
   <body id="{{slug}}" data-spy="scroll" data-target="#navcolumn" data-offset="100">
-{% include "shared/header" %}
+{% include "shared/navbar" %}
 
     <div class="container">
       <div class="row">