I want to make my site editable using a content management system so my client can edit their content

If you are using CloudCannon then just give any element(apart from list (ul,ol and li)) the class of editable. Then under the site settings go to Client Sharing and set a password. Share with you client and your done. If you are using Jekyll then any of your collections can be added to by your client.

If you are using forestry you need to make jekyll fields on anything you want them to be able to change.