Vietnamese translation

Due to the high interest from the community (both internal and external), I've created [this spreadsheet](https://docs.google.com/spreadsheets/d/1K0jK5TbX-__NVjvCJwZXjv7Fht86vu49gpOA38UfWSQ/edit?usp=sharing) to distribute the work.

If anyone is interested in translating, please add your github username to the [People](https://docs.google.com/spreadsheets/d/1K0jK5TbX-__NVjvCJwZXjv7Fht86vu49gpOA38UfWSQ/edit#gid=828191343) sheet and claim the pages you're translating in the [Assignments](https://docs.google.com/spreadsheets/d/1K0jK5TbX-__NVjvCJwZXjv7Fht86vu49gpOA38UfWSQ/edit#gid=0) sheet. Since you don't have editor permission, please leave a comment and I'll add you afterward.

For best practice, please translate by pages broken by `.md` files. This way, the diff won't be as large and will avoid potential merge conflict with others. Also see [this guide](https://google.github.io/eng-practices/review/developer/small-cls.html#what_is_small) for crafting small PRs. Try to avoid making diffs across many `.md` files as such translating the table of content since that will make it difficult to shard out the work among us.

Another way to avoid conflict is to look through the open PRs at https://github.com/google/comprehensive-rust/labels/translation%2Fvi to make sure you don't translate what others have already translated.

If you have any questions, please ask in this issue for transparency.

Happy contributing!