Fixes broken `docs/accounts.html` links in all five migration guides.
## Changes
Replace `https://zed.dev/docs/accounts.html` with
`https://zed.dev/docs/authentication` in:
- `docs/src/migrate/vs-code.md`
- `docs/src/migrate/intellij.md`
- `docs/src/migrate/pycharm.md`
- `docs/src/migrate/webstorm.md`
- `docs/src/migrate/rustrover.md`
These links were flagged as 4xx in an SEO audit. The `accounts.html`
page does not exist; `authentication` is the correct destination.
Release Notes:
- N/A