docs: update stale models docs link in generated config

[minh2416294]

What

Updates the docs link in the generated example config from https://docs.continue.dev/customization/models to
https://docs.continue.dev/customize/models

Why

The "create new assistant" flow (core/config/createNewAssistantFile.ts)
writes an example config.yaml for new users. Its "Define which models can
be used" comment links to /customization/models, which is no longer a
canonical page. It only resolves through the redirect defined in
docs/docs.json (/customization/models → /customize/models). New users
following the link are bounced through a redirect. This points them straight
at the live page.

How

One-line change to the DEFAULT_ASSISTANT_FILE template comment.

Testing

No tests. This is a comment-only string change inside a config template;
there's no behavioral assertion tied to the URL, and the file's formatting is
unchanged (comment text only). Verified locally: git diff --check clean,
Prettier-neutral (only the URL text changed, no structural edits).

Fixes #12669

[minh2416294]

I have read the CLA Document and I hereby sign the CLA

[minh2416294]

fyi the red checks aren't from this pr. its a one line docs comment change in core/config. jetbrains-tests and the win32 vsix build are red on the other open prs too (#12940-12942) so seems like pre-existing ci flakiness, not this. core-checks / lint / prettier / test matrix / cla all green.