Translate docs/tutorial/index.md to pt-br

[iaurg]

Qual o objetivo dessa pull request?

• Adição de uma nova tradução
• Correção em uma tradução existente

Qual arquivo foi traduzido/corrigido?

gatsby-pt-BR/docs/tutorial/index.md

Algum comentário em relação a tradução?

Verifiquem a tradução que fiz de Source Plugin. Quero ver se é a melhor maneira no contexto, procurei no sheet do Google com os termos porém não se adequou ali.

[rafaeelaudibert]

Excelente!

[gatsbot]

Holy buckets, @Italox — we just merged your PR to Gatsby! 💪💜

Gatsby is built by awesome people like you. Let us say “thanks” in two ways:

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