Brief note to maybe spare someone else the trouble. If you want to hide e.g. a huge table in Backstage (techdocs/mkdocs) behind a collapsible element you need the md_in_html extension and use the markdown attribute for it to kick in on the <details> html tag.

Add the extension to your mkdocs.yaml:

markdown_extensions:
  - md_in_html

Hide the table in your markdown document in a collapsible element like this:

<details markdown>
<summary>Long Table</summary>

| Foo | Bar |
|-|-|
| Fizz | Buzz |

</details>

It's also required to have an empty line between the html tag and starting the markdown part. Rendered for me that way in VSCode, GitHub and Backstage.