Make <details>/<summary> folding more discoverable

Since !6568 (merged) we have <details> tag support, but it isn't easy to find out about.

<details>
<summary>Summary lines are collapsible:</summary>
Hiding the details until expanded.
</details>

Ideas for how to make this more discoverable

1. Automatically suggest using <details>/<summary> when the text area reaches a large number of lines
2. Add <details>/<summary> to https://gitlab.com/help/user/markdown
3. Use them in the default issue templates for gitlab-ce
4. Add a button to the noteable toolbar.

Ideas to make more convenient

1. Add custom markdown alternative to <details>, so it can be shorter
2. Render text inside the tag as markdown. This would need changes to redcarpet, see https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/6568#note_16338540 and https://github.com/vmg/redcarpet/issues/13

Related Issues

• https://gitlab.com/gitlab-org/gitlab-ce/issues/21605
• https://gitlab.com/gitlab-org/gitlab-ce/issues/24426

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ORIGINAL PROPOSAL

Sometimes a body of an issue is large, so a user has to scroll an issue down.
It would be great, if there is a markdown code that would hide a text inside it. Like folding in an IDE/text editor.