Document `persistent` key (#2669)
This adds documentation for the `persistent` key, adds it into
documentation examples, and removes `--parallel` where it isn't
necessary. It also updates `examples/` where `dev` tasks are used.
⚠️ This PR should be merged **after** `turbo@1.7` is released to npm.
Stacked PRs:
1. #2546
2. #2258
3. #2669 (this one)
4. #2738
Separate PR so we can merge docs in after the feature has been released.
