Renaming user guide, change logo (#79)

* rename user

* icon

* remove useless img

* update submodule

Author: weiguoz

_data/navigation.yml

@@ -1,5 +1,5 @@
 main:
-  - title: "User Guide"
+  - title: "Language Guide"
     url: sqlflow/doc/language_guide
   - title: "Documents"
     url: doc_index/sqlflow

_layouts/doc.md

@@ -3,7 +3,7 @@ layout: default
 ---
 
 <script>
-    document.getElementsByClassName('site-title')[0].innerHTML = "<img src='/assets/sqlflow-logo.svg' style='height: 30px' />"
+    document.getElementsByClassName('site-title')[0].innerHTML = "<img src='/assets/sqlflow-logo.png' style='height: 30px' />"
 </script>
 
 <style>

_layouts/index.html

@@ -886,7 +886,7 @@
                         <div class="title">
                             <div class="main-nav">
                                 <div class="logo">
-                                    <img alt="logo" src="assets/sqlflow-logo.svg" />
+                                    <img alt="logo" src="assets/sqlflow-logo.png" />
                                 </div>
 
                                 <ul class="inline-list right">