Guide about what requires downtime

## What does this MR do?

This MR adds a guide describing various SQL operations and whether they need downtime or not.

## Are there points in the code the reviewer needs to double check?

Spalling and styling of the text mostly.

## Why was this MR needed?

Developers aren't always aware of the impact of certain operations and the documentation of MySQL and PostgreSQL can be quite confusing at times.

## Screenshots (if relevant)

![screenshot](/uploads/d8afd4bd3755d26e4786dfafecfa9368/screenshot.png)

<img src="https://emoji.slack-edge.com/T02592416/trollface/8c0ac4ae98.png" width="60" />

## Does this MR meet the acceptance criteria?

- [x] ~~[CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added~~
- [x] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- Tests
  - [ ] All builds are passing
- [ ] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [ ] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

See merge request !5672