Refactor Docker docs

What does this MR do?

• Create top level sections and guide the user based on them
• Standardize to 'shell' for all codeblocks
• Use inline links

Since this changes a lot of stuff, it would be easier to see the before and after:

Before	After
https://docs.gitlab.com/omnibus/docker/README.html	http://docs-preview-omnibus-refactor-docker-docs.178.62.207.141.nip.io/omnibus/docker/README.html

Related issues

Closes gitlab#207892 (closed)

Changing the header title?

• Is it completely necessary to change the title? Changing titles breaks deep linking. If yes, proceed further.
• Move the old heading title to the bottom of the page.
• Change the header tag to h5, #####.

Moving docs to a new location?

• Make sure the old link is not removed and has its contents replaced with a link to the new location.
• Make sure internal links pointing to the document in question are not broken.