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Added performance guidelines for new MRs

## What does this MR do?

This MR adds a set of guides that should be followed by merge request authors.

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

Spelling, grammar, etc

## Why was this MR needed?

There is no set of guidelines one should follow when submitting merge requests. This leads to developers at times disregarding performance. This in turn results in performance specialists having to clean up the mess, or production engineers being woken up in the middle of the night because the database is on fire.

## Does this MR meet the acceptance criteria?

- [x] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- Tests
  - [x] All builds are passing
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] 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)

cc @DouweM @rspeicher @pcarranza @dzaporozhets 

See merge request !5905

Clarify blank line rule in newlines_styleguide.md

## What does this MR do?

* Corrects a typo in the GitLab Ruby newlines style guide
* Makes some vague language in the original guideline more precise

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

n/a

## Why was this MR needed?

* There was a typo in the original text.
* The original text was vague.

## What are the relevant issue numbers?

n/a

## Screenshots (if relevant)

n/a

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added  (Is this recommended for this kind of patch?)
- ~~[Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)~~
- ~~API support added~~
- Tests
  - ~~Added for this feature/bug~~
  - [x] All builds are passing
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] 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 !6048

Clarify steps when changing a document's location

Clarify the steps needed to change a document's location so issues like
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/6024
don't occur.

See merge request !6070

[ci skip]

[ci skip]

To clarify what's meant by "from a logical perspective" here, I
consulted Python's PEP8 style guide, which provides some helpfully
precise language: 

> Extra blank lines may be used (sparingly) to separate groups of 
> related functions. Blank lines may be omitted between a bunch of 
> related one-liners (e.g. a set of dummy implementations).

https://www.python.org/dev/peps/pep-0008/#blank-lines

I adapted this passage to the existing language for the newline rule.

[ci skip]

These guidelines cover the performance requirement for newly submitted
merge requests. These guidelines are put in to place to prevent merge
requests from negatively impacting GitLab performance as much as
possible.