ARROW-4481: [Website] Remove generated specification docs from site after docs migration

site/README.md

@@ -44,13 +44,6 @@ subdirectory of `site`.
 git clone --branch=asf-site https://github.com/apache/arrow-site.git asf-site
 ```
 
-Also, from the `site` directory, run the following command to import the format
-markdown documents so that they will rendered.
-
-```
-scripts/sync_format_docs.sh
-```
-
 ## Previewing the site
 
 From the `site` directory, run the following to generate HTML files and run the

site/_config.yml

@@ -30,7 +30,6 @@ include:
 exclude:
   - Gemfile
   - Gemfile.lock
-  - _docs/format/*
   - ruby
   - asf-site
   - scripts

site/_includes/header.html

@@ -35,17 +35,11 @@
             <a class="dropdown-item" href="{{ site.baseurl }}/powered_by/">Powered By</a>
           </div>
         </li>
-        <li class="nav-item dropdown">
-          <a class="nav-link dropdown-toggle" href="#"
-             id="navbarDropdownSpecification" role="button" data-toggle="dropdown"
-             aria-haspopup="true" aria-expanded="false">
+        <li class="nav-item">
+          <a class="nav-link" href="{{ site.baseurl }}/docs/format/README.html"
+             role="button" aria-haspopup="true" aria-expanded="false">
              Specification
           </a>
-          <div class="dropdown-menu" aria-labelledby="navbarDropdownSpecification">
-            <a class="dropdown-item" href="{{ site.baseurl }}/docs/memory_layout.html">Memory Layout</a>
-            <a class="dropdown-item" href="{{ site.baseurl }}/docs/metadata.html">Metadata</a>
-            <a class="dropdown-item" href="{{ site.baseurl }}/docs/ipc.html">Messaging / IPC</a>
-          </div>
         </li>
         <li class="nav-item dropdown">
           <a class="nav-link dropdown-toggle" href="#"