feat(region): add Japan (JP) region support

[Nandu-pns]

Summary

Adds support for New Relic's Japan (JP) data center region across the CLI. All changes are purely additive — no existing behavior for US or EU users is modified.

Changes

Core region support (4 files):

• internal/config/config.go — Added JP to the valid region allow-list (validator is write-only, so existing profiles are unaffected on startup)
• internal/install/command.go — Updated invalid-region error message to include "JP"
• internal/install/execution/platform_link_generator.go — Added one.jp.newrelic.com hostname and JP branch in
  nrPlatformHostname() (US fallback unchanged)
• internal/profile/command.go — Added "JP" to interactive region selector and --region flag description

Documentation (5 files):

• docs/GETTING_STARTED.md
• docs/cli/newrelic_profile_add.md
• internal/migrate/tf_importgen_guide.md
• internal/migrate/tf_importgen_ci_guide.md
• internal/fleetcontrol/README.md

Dependency

⚠️ This PR depends on newrelic/newrelic-client-go#1391 which adds the JP region constant and endpoint to the client library. Do not merge until that PR is merged, a new version is tagged, and go.mod is bumped + vendored in this repo.