Cleanup of docs/design directory

[loicraux]

Have you read the FAQ and checked for duplicate open issues?
Yes

Is your feature request related to a problem? Please describe.
Understanding the design of Shaka Player in order to be able to contribute with PRs that meet quality & design requirements can be difficult for newcomers. I see that there are design documents under docs/design folder but these documents are between 2 and 7 years old, so I'm not sure if they're up to date when I read them...

Describe the solution you'd like
It would be great if the design documents under docs/design were arranged in different sub-folders. I propose the following sub-folders for instance :

• docs/design/ongoing_work for design proposals related to ongoing work (for a future next major release ?). Content is meant to be moved to docs/design/current once related design has been shipped, or deleted is feature/design has indeed been dropped.
• docs/design/current for design docs up-to-date with the current design of Shaka Player
• docs/design/outdated for legacy documents that are not up to date with the current Shaka Player design, but which are considered to remain worthy of interest.

Describe alternatives you've considered
I couldn't think of any other way to clean up the docs/design folder, but any solution that makes it easier to familiarize yourself with the current Shaka Player design thanks to the existing documentation is welcome.

Additional context
#1301 (comment)