Restructure AI docs (#57614)

## Summary

- Restructures the AI docs around quick routing, agent paths, provider
access, and company-specific entry points.
- Moves account-owned AI billing/model pages into Account & Billing
while preserving legacy AI redirects.
- Adds focused pages for Zed Agent, external agents, terminal threads,
agent profiles, skills, instructions, API access, subscriptions,
gateways, and local models.
- Updates related links across Git, authentication, business, migration,
and settings reference docs.

## Validation

- `pnpm dlx prettier@3.5.0 . --write`
- `pnpm dlx prettier@3.5.0 . --check`
- `git diff --check -- docs/src`
- Changed-doc local markdown link sanity check
- `mdbook build docs` was attempted; it is currently blocked by an
unrelated existing `docs/src/vim.md:542` action schema error for
`vim::HelixJumpToWord`.

## Follow-up

- Create a ticket to align Zed's in-app AI configuration surfaces with
this documentation model, including Agent Settings, Configure buttons,
Tools, Profiles, MCP, Skills/Instructions, and plain settings.

## Suggested .rules additions

N/A

Release Notes:

- N/A

docs/src/migrate/vs-code.md

@@ -329,9 +329,9 @@ Once signed in, just start typing. Zed will offer suggestions inline for you to
 
 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/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).
+- Use Zed’s hosted models, with higher rate limits. Requires [authentication](https://zed.dev/docs/authentication) and access through [Zed Pro](https://zed.dev/docs/account/zed-hosted-models.html).
+- Bring your own [API keys](https://zed.dev/docs/ai/use-api-access.html), no authentication needed
+- Use [External Agents like Claude Agent](https://zed.dev/docs/ai/external-agents.html).
 
 ### Advanced Config and Productivity Tweaks