Delete doc since it was broken into smaller docs (gatsbyjs#9262)

This is a proposal to delete this doc now that we have broken all the deployment options into separate docs and I just added them to the sidebar (commit gatsbyjs@1af244b). If this PR gets merged I'll also delete this doc from the sidebar.

Taken from gatsbyjs#8103: 

### Deploying
- [x] Deploying to S3/Cloudfront
- [x] Deploying to Netlify
- [x] Deploying to Aerobatic gatsbyjs#8736
- [x] Deploying to Heroku gatsbyjs#8721 (PR)
- [x] Deploying to Now gatsbyjs#8737
- [x] Deploying to GitLab Pages gatsbyjs#8738

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