Translate what-is-a-plugin to pt-br

[CarlosEduLima]

Qual o objetivo dessa pull request?

• [x ] Adição de uma nova tradução
• Correção em uma tradução existente

Qual arquivo foi traduzido/corrigido?

what-is-a-plugin.md
https://github.com/gatsbyjs/gatsby-pt-BR/blob/master/docs/docs/what-is-a-plugin.md

Algum comentário em relação a tradução?

[jessescn]

Tradução excelente @CarlosEduLima. Não tenho nada a adicionar/apontar, por mim ta aprovado ✔️

[rafaeelaudibert]

Excelente tradução, @CarlosEduLima!

Esperamos que possa contribuir novamente com a gente! ✨

[gatsbot]

Holy buckets, @CarlosEduLima — 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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