message: Update videos.md with architecture walkthrough
diff: # Videos
  -There are currently two videos about Salsa available, but they describe an older version of Salsa and so they are rather outdated:
  +There is currently one video available on the newest version of Salsa:
  -- [How Salsa Works](https://youtu.be/_muY4HjSqVw), which gives a
  - high-level introduction to the key concepts involved and shows how
  - to use Salsa;
  -- [Salsa In More Depth](https://www.youtube.com/watch?v=i_IhACacPRY),
  - which digs into the incremental algorithm and explains -- at a
  - high-level -- how Salsa is implemented.
  +- [Salsa Architecture Walkthrough](https://www.youtube.com/watch?v=vrnNvAAoQFk),
  + which covers many aspects of the redesigned architecture.
  -> If you're in China, watch videos on [How Salsa Works](https://www.bilibili.com/video/BV1Df4y1A7t3/), [Salsa In More Depth](https://www.bilibili.com/video/BV1AM4y1G7E4/).
  +There are also two videos on the older version Salsa, but they are rather
  +outdated:
  +
  +- [How Salsa Works](https://youtu.be/_muY4HjSqVw), which gives a high-level
  + introduction to the key concepts involved and shows how to use Salsa;
  +- [Salsa In More Depth](https://www.youtube.com/watch?v=i_IhACacPRY), which digs
  + into the incremental algorithm and explains -- at a high-level -- how Salsa is
  + implemented.
  +
  +> If you're in China, watch videos on
  +> [How Salsa Works](https://www.bilibili.com/video/BV1Df4y1A7t3/),
  +> [Salsa In More Depth](https://www.bilibili.com/video/BV1AM4y1G7E4/).
  