docs: add username policy guide

[simeng-li]

Summary

Documents the per-tenant username policy feature shipped in logto-io/logto#8991 (LOG-13621).

What changed

• New page docs/end-user-flows/sign-up-and-sign-in/username-policy.mdx — modeled on the password-policy page: baseline rules vs per-tenant policy, enforcement surfaces (end-user flows always enforce; Management API applies baseline only), Console setup steps (Advanced options → Username policy → Manage: case sensitivity, length, allowed characters), and the case-insensitive switch flow (409 guard + proactive conflict detection with a link to the GET /api/sign-in-exp/username-policy/case-sensitivity-conflicts operation). Sidebar position 7; displaced pages bumped (disable-user-registration → 8, error-pages → 9).
• user-data.mdx §username — "It's case-sensitive." became "case-sensitive by default" with a link to the policy page, since case sensitivity/length/charset are now per-tenant configurable.
• user-data.mdx §profile — documents the new preferred_username claim fallback to username when profile.preferredUsername is unset (explicit value wins).
• sign-up.mdx — username identifier bullet links to the policy page.

Testing

N/A

Checklist

• .changeset
• unit tests
• integration tests
• necessary TSDoc comments

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