Docs organize main page by product category

See the general Documentation guidelines http://docs.gitlab.com/ce/development/doc_styleguide.html

What does this MR do?

Organizes https://docs.gitlab.com/ce/README.html by categories (stages of the dev cycle)

Review apps:

• CE: http://ce-docs-organize-by-category.178.62.207.141.xip.io/ce
• EE: http://ee-docs-organize-by-category-ee.178.62.207.141.xip.io/ee

Notes:

• When we switch to Kramdown, we can easily add the corresponding icon to the headers, like this:

We can't do this now bc of the ToC. It would be easily solved with Kramdown custom classes.

EE-version: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/5052