Improve readability of markdown files

What does this MR do?

It improves readability of markdown files by:

• Adjusting relative header sizes and margins (the same as on github)
• Adding 30px at the top and the bottom of markdown files
• Reducing ul/ol margin by 3px
• Displaying images as inline-block (see GitHub Changelog Generator for fixed layout)
• Adding horizontal line also below h2 tag
• Capping page width to 840px and centre it

Why was this MR needed?

Current markdown layout is quite painful to read, especially if someone is used to arguably pretty GitHub formatting.

What are the relevant issue numbers?

• https://gitlab.com/gitlab-org/gitlab-ce/issues/17366
• probably more, but there are too many of them to search

Screenshots (if relevant)

• changelog-before
• changelog-after
• gitlab-before
• gitlab-after
• gaze-before
• gaze-after
• pageres-before
• pageres-after

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together