Add guide on changing a document's location
## What does this MR do?
Add a documentation styleguide when changing a document's location.
## What are the relevant issue numbers?
https://gitlab.com/gitlab-org/gitlab-ce/issues/3349
See merge request !4624
- Fix some syntax/grammar typos
- Link to GFM documentation on newlines
- Be less strict on the alphabetical order styleguide
- You can override the "numbers in headings" rule if you discuss it first
- Do not mention CE in notes if the feature is in both CE and EE