Fix broken accounts.html links in migrate docs (#49744)

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

docs/src/migrate/vs-code.md

@@ -334,7 +334,7 @@ To invoke completions, just start typing. Zed will offer suggestions inline for
 
 To use other AI models in Zed, you have several options:
 
-- Use Zed’s hosted models, with higher rate limits. Requires [authentication](https://zed.dev/docs/accounts.html) and subscription to [Zed Pro](https://zed.dev/docs/ai/subscription.html).
+- Use Zed’s hosted models, with higher rate limits. Requires [authentication](https://zed.dev/docs/authentication) and subscription to [Zed Pro](https://zed.dev/docs/ai/subscription.html).
 - Bring your own [API keys](https://zed.dev/docs/ai/llm-providers.html), no authentication needed
 - Use [external agents like Claude Agent](https://zed.dev/docs/ai/external-agents.html).