doc: use a details tag for completed initiatves

The list of completed initiatives is longer and visually more noticeable
than the list of current initiatives, but the current initiatives are
likely the ones of interest. The completed initiatives list is only
going to get longer and longer, so let's put it in a details tag.

doc/guides/strategic-initiatives.md

@@ -17,7 +17,10 @@ are active and have the support needed.
 | Startup performance       | [Joyee Cheung][joyeecheung] | <https://github.com/nodejs/node/issues/17058> <https://github.com/nodejs/node/issues/21563>  |
 | V8 Currency               | [Michaël Zasso][targos]     |                                                                                          |
 
-## Completed
+<details>
+<summary>List of completed initiatives</summary>
+
+## Completed initiatives
 
 | Initiative         | Champion                   | Links                                           |
 |--------------------|----------------------------|--------------------------------------------------|
@@ -33,6 +36,8 @@ are active and have the support needed.
 | VM module fix      | Franziska Hinkelmann       | <https://github.com/nodejs/node/issues/6283>      |
 | Workers            | Anna Henningsen            | <https://github.com/nodejs/worker>                |
 
+</details>
+
 [jasnell]: https://github.com/jasnell
 [joyeecheung]: https://github.com/joyeecheung
 [mcollina]: https://github.com/mcollina