Fixed website generate site (apache#6026)

### Motivation

yarn has been not installed

```
internal/modules/cjs/loader.js:638
    throw err;
    ^

Error: Cannot find module 'marked'
    at Function.Module._resolveFilename (internal/modules/cjs/loader.js:636:15)
    at Function.Module._load (internal/modules/cjs/loader.js:562:25)
    at Module.require (internal/modules/cjs/loader.js:692:17)
    at require (internal/modules/cjs/helpers.js:25:18)
    at Object.<anonymous> (/pulsar/site2/website/brodocs/brodoc.js:3:16)
    at Module._compile (internal/modules/cjs/loader.js:778:30)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:789:10)
    at Module.load (internal/modules/cjs/loader.js:653:32)
    at tryModuleLoad (internal/modules/cjs/loader.js:593:12)
    at Function.Module._load (internal/modules/cjs/loader.js:585:3)
c
```

### Modifications

* Change order for generate admin document

site2/tools/build-site.sh

@@ -40,6 +40,10 @@ yarn build
 
 node ./scripts/replace.js
 
+# Generate document for command line tools.
+${ROOT_DIR}/site2/tools/pulsar-admin-doc-gen.sh
+cd ${ROOT_DIR}/site2/website
+
 rm -rf ${ROOT_DIR}/generated-site/content
 mkdir -p ${ROOT_DIR}/generated-site/content
 cp -R ${ROOT_DIR}/generated-site/api ${ROOT_DIR}/generated-site/content

site2/tools/generate-api-docs.sh

@@ -39,6 +39,3 @@ cd $SCRIPT_DIR
 ./javadoc-gen.sh
 
 ./python-doc-gen.sh
-
-# Generate document for command line tools.
-./pulsar-admin-doc-gen.sh