docs(gatsby-source-wordpress): adapted section about self-signed certificates (#10962)
docs(gatsby-source-wordpress): adapted section about self-signed certificates (#10962)

## Description

To keep `gatsby-node.js` clean and tidy, `NODE_TLS_REJECT_UNAUTHORIZED` should be moved to `.env.development` – where all the other environment variables for development should be too.

Additionally, added a hint about caution for setting this in production.
gatsbyjs/gatsby,gatsbyjs/gatsby,gatsbyjs/gatsby,ChristopherBiscardi/gatsby,ChristopherBiscardi/gatsby,gatsbyjs/gatsby,ChristopherBiscardi/gatsby,ChristopherBiscardi/gatsby,gatsbyjs/gatsby,gatsbyjs/gatsby