docs: refresh custom modes guidance (#5669)

[EloiRamos]

Fixes #5669.

Updates the Custom Modes documentation to match the current UI:

• Clarifies that Modes are accessed from the Prompts tab (notebook icon)
• Replaces outdated “Modes view” references
• Makes the Importing a Mode steps sequential and explicit

[changeset-bot]

⚠️ No Changeset found

Latest commit: d78cc1f

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

This PR includes no changesets

When changesets are added to this PR, you'll see the packages that this PR includes changesets for and the associated semver types

Click here to learn what changesets are, and how to add one.

Click here if you're a maintainer who wants to add a changeset to this PR