Merge branch 'fix/update-memory-requirements' into 'master'

Update memory requirements docs

## What does this MR do?
It doubles the memory requirements in the docs.

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

## Why was this MR needed?
To reflect the requirements for decent operation, until we bring such requirements down.

## What are the relevant issue numbers?
https://gitlab.com/gitlab-com/infrastructure/issues/19

## Screenshots (if relevant)
N/A

## Does this MR meet the acceptance criteria?

- [] ~~[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)
- [ ] ~~API support added~~
- ~~Tests~~
  - [ ] ~~Added for this feature/bug~~
  - [ ] ~~All builds are passing~~
- [ ] ~~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 !6016