Add GSA-TTS prerequisite to Cloud.gov Pages access

[aduth]

Being able to manage static sites either via Cloud.gov Pages or Netlify CMS requires additional permissions to the GSA-TTS GitHub organization. This is a common source of confusion.

The goal of this pull request is to add additional text to the Cloud..gov Pages handbook article to explain this prerequisite, and link to existing guidance on requesting access to be added to the relevant GitHub teams.

[akhlaqkhan]

From what i understand from the slack conversation with GitHub admins, is having “write” permission on GitHub repository for identity-site, as another pre-requisite.

[aduth]

Being part of the identity-core team gives you write access to identity-site, along with other sites.

You can confirm this on the access management page for each repository: https://github.com/GSA-TTS/identity-site/settings/access