Add Netlify CMS admin to codebase

Why is this change being made?

This MR adds the NeltifyCMS code to our repository and makes the admin accessible during local development. This will not make the admin accessible on the live site.

What is it

Netlify CMS is a git based headless CMS system. https://www.netlifycms.org/

Where is it?

'sites/marketing/source/admin/' The config.yml files sets all the configurations for Netlify CMS. This is where you define what content is editable. https://www.netlifycms.org/docs/configuration-options/#configuration-file

Accessing the Admin

As of now, you can only access Netlify CMS while working locally with the repository. https://www.netlifycms.org/docs/beta-features/#working-with-a-local-git-repository

1. Run npx netlify-cms-proxy-server from the sites/marketing directory of the above repository.
2. In seperate window, fire up local server of marketing site bundle exec middleman serve
3. Visit http://localhost:4567/admin/

Related https://gitlab.com/gitlab-com/marketing/growth-marketing/growth/-/issues/622

Author Checklist

• Provided a concise title for the MR
• Added a description to this MR explaining the reasons for the proposed change, per say-why-not-just-what
• Assign this change to the correct DRI
  • If the DRI for the page/s being updated isn’t immediately clear, then assign it to your manager.
  • If your manager does not have merge rights, please ask someone to merge it AFTER it has been approved by your manager in #mr-buddies.
  • If the changes relate to any part of the project other than updates to content and/or data files please make sure to ping @gl-static-site-editor in a comment for a review and merge. For example changes to .gitlab-ci.yml, JavaScript/CSS/Ruby code or the layout files.