Change docs link for Wiki Markdown and improve docs

What does this MR do and why?

UI:

• Fix help link on wiki edit view to point to https://docs.gitlab.com/user/project/wiki/markdown/. The previous link points at a non-existing anchor https://docs.gitlab.com/user/markdown/#wiki-specific-markdown.
  • The link now doesn't depend on generic markdownHelpPath with a fragment.
• Edit UI text so it doesn't say simply.
• Use link text that follows https://design.gitlab.com/patterns/contextual-help/#link-text.

Docs:

• Explain what the page slug is.
• Document all link types available in wikis in https://docs.gitlab.com/user/project/wiki/markdown/. Previously, https://docs.gitlab.com/user/markdown/#gitlab-specific-references was the only place where you could learn about [[wiki-style]] links.

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. On the left sidebar, select Search or go to and find your project or group.
2. Select Plan > Wiki.
3. Select Wiki actions (three dots), then New page on this page, or any other wiki page.

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