George Kalpakas e5b4b83778 fix(docs-infra): improve styling of completed Roadmap projects (#42620)
This commit improves the styling of the "Completed projects" section of
the Roadmap by (a) making it consistent with the rest of the Roadmap
sections and (b) making it more similar with `<details>` elements on
other pages.

**Before:**
  _Collapsed:_ ![completed projects collapsed (before)][1]
  _Expanded:_ ![completed projects expanded (before)][2]
**After:**
  _Collapsed:_ ![completed projects collapsed (after)][3]
  _Expanded:_ ![completed projects expanded (after)][4]

[1]: https://user-images.githubusercontent.com/8604205/122917101-50c2ca00-d366-11eb-8348-01efd69cedf2.png
[2]: https://user-images.githubusercontent.com/8604205/122917109-515b6080-d366-11eb-9f18-c794fcea8dd1.png
[3]: https://user-images.githubusercontent.com/8604205/122917115-528c8d80-d366-11eb-947a-2d7da3950069.png
[4]: https://user-images.githubusercontent.com/8604205/122917117-53252400-d366-11eb-93e7-9384a7431c0d.png

PR Close #42620
2021-06-28 09:33:17 -07:00
..
2021-05-27 08:55:28 -07:00
2021-02-22 12:37:40 -08:00