This fixes two broken links in the performance guide and removes the
mention of Josh as he no longer works for GitLab.

[ci skip]

Mention add_column_with_default in downtime guide

## What does this MR do?

This extends the "What causes downtime" guide with a mention of the `add_column_with_default` migration helper.

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

Spelling, grammar, etc.

## Why was this MR needed?

Currently the guide may lead one to believe it's not possible at all to add a column with a default value.

## 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 !5719

Update UI Guide with SVG guidelines

This addition to the guide is to provide some guidelines to UX designers when exporting SVGs. Please let me know if anything is unclear or if you any improvements so we can document it clearly for everyone.

cc / @hazelyang @cperessini @dimitrieh @connorshea @annabeldunstone @dzaporozhets 

## What are the relevant issue numbers?
https://gitlab.com/gitlab-org/gitlab-ee/issues/872

See merge request !5748

- Fix wrong doc_styleguide URL
- Clarify migrations refer to SQL
- Capitalize CSS

[ci skip]

Link to doc root, not readme of GDK

I hope this makes navigation between gitlab-ce developer docs and GDK docs more smooth.

See merge request !5716

Use long options for curl examples in documentation

## What does this MR do?

Use long options (e.g. `--header` instead of `-H`) for curl examples in documentation.

## Why was this MR needed?

Short options are less readable.

## What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/5465#note_13603730


See merge request !5703