Add documentation on secrets management for static sites deployed on Netlify

[hancush]

Documentation request

tl;dr - Environment variables

• https://www.gatsbyjs.org/docs/environment-variables/
• https://docs.netlify.com/configure-builds/environment-variables/#declare-variables

[smcalilly]

would we want do document anything more than pointing to the existing docs? if not then we can close this.

[hancush]

Pointing to existing docs would be great.