Translate docs/api-reference and docs/theme-api

[AdamSiekierski]

Take a look at some words like "plugin" or the links I've included (left the English ones, as the docs still hasn't got the Polish) :)

[gatsbot]

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

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

1. We’d like to send you some Gatsby swag. As a token of our appreciation, you can go to the Gatsby Swag Store and log in with your GitHub account to get a coupon code good for one free piece of swag. We’ve got Gatsby t-shirts, stickers, hats, scrunchies, and much more. (You can also unlock even more free swag with 5 contributions — wink wink nudge nudge.) See gatsby.dev/swag for details.
2. We just invited you to join the Gatsby organization on GitHub. This will add you to our team of maintainers. Accept the invite by visiting https://github.com/orgs/gatsbyjs/invitation. By joining the team, you’ll be able to label issues, review pull requests, and merge approved pull requests.

If there’s anything we can do to help, please don’t hesitate to reach out to us: tweet at @gatsbyjs and we’ll come a-runnin’.

Thanks again!

[maciekgrzybek]

@AdamSiekierski swag swag :D