Demonstrate <details> and <summary> tags in doc/user/markdown.md

What

Adds examples of HTML <details> and <summary> tags to https://gitlab.com/help/user/markdown as mentioned in https://gitlab.com/gitlab-org/gitlab-ce/issues/15230#note_26802849.

Why

So people who don't know what they are can discover their support. Also demonstrates use of <pre><code> tags as an alternative to ``` code fences within HTML

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated

Related

• https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/6568
• https://gitlab.com/gitlab-org/gitlab-ce/issues/15230