Central location for gatsby-* environment variables

[M4ss1ck]

I would like to have a reference page for all GATSBY-* environmental variables, sort of a central location for them.

The how-to about environment variables shows us how to access them in the browser, the ones specific to Gatsby Cloud, additional ones like STAGING and TEST and reserved ones (NODE_ENV, PUBLIC_DIR, ...).

Even in small projects, it's easy to realize we're using many gatsby-* plugins, each of them with a few potential specific environment variables. E.g: gatsby-source-filesystem has GATSBY_CONCURRENT_DOWNLOAD, GATSBY_STALL_RETRY_LIMIT, GATSBY_STALL_TIMEOUT, GATSBY_CONNECTION_TIMEOUT.

Perhaps they are not that many as I could think, but a central place where you could keep track of all this third party environment variables and their descriptions seems like a good idea, at least for avoiding collisions between different plugins when naming them.

PS: blame @AskGatsbyJS for this