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

We no longer need to render the format docs as part of the Jekyll site, as they were ported to the master Sphinx project. As a result of this, the instructions to build the website were broken

Author: Wes McKinney <wesm+git@apache.org>

Closes #3620 from wesm/ARROW-4481 and squashes the following commits:

013d13bf <Wes McKinney> Remove generated specification docs from site after docs migration
diff --git a/README.md b/README.md
index 7a287ec..73fd185 100644
--- a/README.md
+++ b/README.md
@@ -44,13 +44,6 @@
 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
diff --git a/_config.yml b/_config.yml
index b7ec25b..ae420e4 100644
--- a/_config.yml
+++ b/_config.yml
@@ -30,7 +30,6 @@
 exclude:
   - Gemfile
   - Gemfile.lock
-  - _docs/format/*
   - ruby
   - asf-site
   - scripts
diff --git a/_docs/ipc.md b/_docs/ipc.md
deleted file mode 100644
index bc22dc3..0000000
--- a/_docs/ipc.md
+++ /dev/null
@@ -1,25 +0,0 @@
----
-layout: docs
-title: Arrow Messaging and IPC
-permalink: /docs/ipc.html
----
-<!--
-{% comment %}
-Licensed to the Apache Software Foundation (ASF) under one or more
-contributor license agreements.  See the NOTICE file distributed with
-this work for additional information regarding copyright ownership.
-The ASF licenses this file to you under the Apache License, Version 2.0
-(the "License"); you may not use this file except in compliance with
-the License.  You may obtain a copy of the License at
-
-http://www.apache.org/licenses/LICENSE-2.0
-
-Unless required by applicable law or agreed to in writing, software
-distributed under the License is distributed on an "AS IS" BASIS,
-WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-See the License for the specific language governing permissions and
-limitations under the License.
-{% endcomment %}
--->
-
-{% include_relative format/IPC.md %}
\ No newline at end of file
diff --git a/_docs/memory_layout.md b/_docs/memory_layout.md
deleted file mode 100644
index 74cd7ed..0000000
--- a/_docs/memory_layout.md
+++ /dev/null
@@ -1,25 +0,0 @@
----
-layout: docs
-title: Physical Memory Layout
-permalink: /docs/memory_layout.html
----
-<!--
-{% comment %}
-Licensed to the Apache Software Foundation (ASF) under one or more
-contributor license agreements.  See the NOTICE file distributed with
-this work for additional information regarding copyright ownership.
-The ASF licenses this file to you under the Apache License, Version 2.0
-(the "License"); you may not use this file except in compliance with
-the License.  You may obtain a copy of the License at
-
-http://www.apache.org/licenses/LICENSE-2.0
-
-Unless required by applicable law or agreed to in writing, software
-distributed under the License is distributed on an "AS IS" BASIS,
-WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-See the License for the specific language governing permissions and
-limitations under the License.
-{% endcomment %}
--->
-
-{% include_relative format/Layout.md %}
\ No newline at end of file
diff --git a/_docs/metadata.md b/_docs/metadata.md
deleted file mode 100644
index 382ab0ea..0000000
--- a/_docs/metadata.md
+++ /dev/null
@@ -1,25 +0,0 @@
----
-layout: docs
-title: Arrow Metadata
-permalink: /docs/metadata.html
----
-<!--
-{% comment %}
-Licensed to the Apache Software Foundation (ASF) under one or more
-contributor license agreements.  See the NOTICE file distributed with
-this work for additional information regarding copyright ownership.
-The ASF licenses this file to you under the Apache License, Version 2.0
-(the "License"); you may not use this file except in compliance with
-the License.  You may obtain a copy of the License at
-
-http://www.apache.org/licenses/LICENSE-2.0
-
-Unless required by applicable law or agreed to in writing, software
-distributed under the License is distributed on an "AS IS" BASIS,
-WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-See the License for the specific language governing permissions and
-limitations under the License.
-{% endcomment %}
--->
-
-{% include_relative format/Metadata.md %}
\ No newline at end of file
diff --git a/_includes/header.html b/_includes/header.html
index 2abf60f..39a1d05 100644
--- a/_includes/header.html
+++ b/_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="#"