Fix markdown help references

## What does this MR do?
Replaces all `markdown/markdown` references with `user/markdown` since `markdown/markdown.md` has been replaced with `user/markdown.md`

## Are there points in the code the reviewer needs to double check?
Shouldn't be any 😄 

## Why was this MR needed?
So that users who click on markdown help don't have to click/redirect to another link to get to the markdown help page

## What are the relevant issue numbers?
Closes #21434

## Screenshots (if relevant)
```
$git grep markdown/markdown
app/helpers/search_helper.rb:47:      { category: "Help", label: "Markdown Help",      url: help_page_path("markdown/markdown") },
app/views/admin/appearances/_form.html.haml:16:        Description parsed with #{link_to "GitLab Flavored Markdown", help_page_path('markdown/markdown'), target: '_blank'}.
app/views/help/ui.html.haml:552:  You can check how markdown rendered at #{link_to 'Markdown help page', help_page_path("markdown/markdown")}.
doc/development/doc_styleguide.md:425:[gfm]: http://docs.gitlab.com/ce/markdown/markdown.html#newlines "GitLab flavored markdown documentation"
doc/user/markdown.md:69:https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/markdown/markdown.md#newlines
doc/user/markdown.md:89:https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/markdown/markdown.md#multiple-underscores-in-words
doc/user/markdown.md:104:https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/markdown/markdown.md#url-auto-linking
doc/user/markdown.md:125:https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/markdown/markdown.md#multiline-blockquote
doc/user/markdown.md:159:https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/markdown/markdown.md#code-and-syntax-highlighting
doc/user/markdown.md:229:https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/markdown/markdown.md#inline-diff
doc/user/markdown.md:245:https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/markdown/markdown.md#emoji
doc/user/markdown.md:310:https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/markdown/markdown.md#task-lists
doc/user/markdown.md:333:https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/markdown/markdown.md#videos
doc/user/markdown.md:783:[markdown.md]: https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/markdown/markdown.md
spec/routing/routing_spec.rb:110:    path = '/help/markdown/markdown.md'
spec/routing/routing_spec.rb:112:                                  path: 'markdown/markdown',
```

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- 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)

See merge request !6024