[WIP][翻訳][Part3]42ad0ad4c5まで更新

Part3を英語版の https://github.com/dsyph3r/symblog-docs/commit/15d15cee5ccf72eca1708e3d5d8e0ef9ffdeb207#diff-827edf1bcdceb9b79f1e6db4e030bec7 まで更新。

```
git diff e1737d5f4c1103a5c182c29ad44dd617d572f4f1..HEAD docs/doctrine-2-the-blog-model.rst
```