Update build site instruction with python3.

Former-commit-id: 6c2655854e30725999dd308c8524e3daf234a2b9

.gitignore

@@ -100,3 +100,4 @@ python/buildstatic
 tutorial/cmake-build-debug
 .vscode/
 .idea/
+site/

mkdocs.yml

@@ -78,7 +78,7 @@ pages:
       - rbr: file-formats/rbr.md
       - tgf: file-formats/tgf.md
       - xml: file-formats/xml.md
-    - Building the Website: building-the-website.md
+    - Building the Website: README.md
   - FAQ: faq.md
   - About:
     - Release History: RELEASE_HISTORY.md