Add padding to details element in markdown

What does this MR do?

Fixes #34338 (moved).

Here are the requests in the issue:

1. There should be some space at the bottom of the details element, so it's not so tight against whatever comes next.
2. We should parse markdown (in the gif below, there's an unordered list) inside the details element, see https://gitlab.com/gitlab-org/gitlab-ce/issues/30846
3. The focus on the summary toggle shouldn't persist -- it looks weird

Note:

• In this MR, 1 gets fixed, giving space on the last element, as well as the first element in the <details> element.
• 2 is already fixed, we parse markdown in there now.
• WONTFIX 3. This outline is for accessibility purposes.

Screenshots

Before	After

Does this MR meet the acceptance criteria?

This also accounts for cases where a <details> element might contain a <summary> element, which you may or may not have in some cases.

Conformity

• Changelog entry for user-facing changes, or community contribution. Check the link for other scenarios.
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Closes #34338 (moved)