Write more FAQ answers based on feedback

The following questions are some ideas of what questions could be added to the FAQ page based on feedback we've received on the page so far.

🙋🏼 If anyone wants to take one (or more), please comment in this issue and then [follow the contributing guidelines](https://github.com/graphql/graphql.github.io/blob/source/CONTRIBUTING.md#add-a-question-to-the-faq) to open your pull request 🚀

Here are the new questions added (sorted by section):

**Getting Started**
- [ ] Can I use GraphQL to make graphs or charts?
- [ ] What are the downsides of GraphQL?

**General**
- [ ] Does GraphQL only work with graph-like structures or data sources that are graph-based?

**Best Practices**
- [ ] Does GraphQL introduce a single point of failure?
- [ ] How can I make part of my GraphQL API private?
- [ ] How can I do error handling?
- [ ] Schema-first or code-first?

**Specification**
- [ ] Why doesn’t GraphQL have a wildcard field?
- [ ] What is the N+1 problem I keep hearing about?

issueから未チェックの質問を1つ選び、リポジトリ内でFAQページを探す前に、リンク先の「add a question to the FAQ」セクションにあるCONTRIBUTING.mdを読んでください。文書化されたコントリビューションプロセスに従い、調査した回答を追加し、pull requestを作成する前に、選択した質問が明確に回答されていることを確認してください。

索引モデルが issue の本文から書いたものです。