Update docs for github.com being down
Update docs for github.com being down

Disabling security is trivial in newer versions of Jenkins, so this
should be our emergency process if github.com is down. The Jenkins UI
should still be protected by IP whitelisting.

alphagov/govuk-developer-docs,alphagov/govuk-developer-docs,alphagov/govuk-developer-docs,alphagov/govuk-developer-docs