Add starter template tutorial for Bitbucket

Bitbucket's permissions don't allow deploy keys with write permission, which means that people can't create Bitbucket repos with a "Deploy to Netlify" button. Since this leaves Bitbucket users out from the [Start with a Template](https://www.netlifycms.org/docs/start-with-a-template/) page, it would be nice to be able to link to a tutorial on the Netlify CMS blog.

It could follow these steps:

1. Clone a starter repo to Bitbucket. The post could include both the Hugo and Gatsby starters from the templates page, since setup will be the same for either. The repos should be linked directly, rather than in Deploy to Netlify buttons.
2. Link the repository to Netlify (like in the video [here](https://www.netlify.com/docs/continuous-deployment/)).
3. Follow Step 1 of the [Git Gateway backend setup](https://deploy-preview-1512--netlify-cms-www.netlify.com/docs/authentication-backends/#git-gateway-with-netlify-identity). (Step 2 will already be done in the repo, and step 3 is optional.)
4. Continue with the steps to [access Netlify CMS on your new site](https://www.netlifycms.org/docs/start-with-a-template/#access-netlify-cms-on-your-new-site) from the "Start with a Template" page.