Docs: Rename bootstrap-theme.css to bootstrap-gohugo.css

because our bootstrap-theme.css was originally a customized
core bootstrap.css file from Bootstrap v3.0.0.

This rename helps to avoid confusion with Bootstrap’s official
bootstrap-theme.css files.

docs/layouts/index.html

@@ -9,7 +9,7 @@
     <title>Hugo :: A fast and modern static website engine</title>
 
     <!-- Bootstrap core CSS -->
-    <link href="/css/bootstrap-theme.css" rel="stylesheet">
+    <link href="/css/bootstrap-gohugo.css" rel="stylesheet">
 
     <link href='//fonts.googleapis.com/css?family=Arbutus+Slab%7CCabin:600%7CSource+Code+Pro' rel='stylesheet' type='text/css'>
     <!-- Add custom CSS here -->

docs/layouts/partials/header.html

@@ -10,7 +10,7 @@
 {{ .Scratch.Add "title" "" }}{{ if isset .Site.Data.titles .Title }}{{ .Scratch.Set "title" (index .Site.Data.titles .Title).title }}{{ else }}{{ .Scratch.Set "title" .Title}}{{end}}
     <title>Hugo - {{ .Scratch.Get "title" }}</title>
 
-    <link href="/css/bootstrap-theme.css" rel="stylesheet">
+    <link href="/css/bootstrap-gohugo.css" rel="stylesheet">
     <link href="/assets/font-awesome/css/font-awesome.min.css" rel="stylesheet" />
     <link href="/css/style.css" rel="stylesheet">
     <link href="/css/style-responsive.css" rel="stylesheet" />