docs: add backstage-cli usage guide for RHDH

[kadel]

Document how to use @backstage/cli auth and actions commands with RHDH, including auth plugin setup, OAuth2 endpoint configuration, standalone usage workaround for npm peer dependency conflicts, and troubleshooting.

Assisted-by: Claude Code

Description

Please explain the changes you made here.

Which issue(s) does this PR fix

related to https://redhat.atlassian.net/browse/RHIDP-14120

PR acceptance criteria

Please make sure that the following steps are complete:

• GitHub Actions are completed and successful
• Unit Tests are updated and passing
• E2E Tests are updated and passing
• Documentation is updated if necessary (requirement for new features)
• Add a screenshot if the change is UX/UI related

How to test changes / Special notes to the reviewer

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 52.86%. Comparing base (105732d) to head (b8506ea).
⚠️ Report is 27 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4901      +/-   ##
==========================================
- Coverage   53.65%   52.86%   -0.80%     
==========================================
  Files         121      109      -12     
  Lines        2350     2132     -218     
  Branches      539      537       -2     
==========================================
- Hits         1261     1127     -134     
+ Misses       1084     1004      -80     
+ Partials        5        1       -4     

Flag	Coverage Δ
rhdh	52.86% <ø> (-0.80%)	⬇️

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[github-actions]

The container image build workflow finished with status: cancelled.

[github-actions]

The container image build workflow finished with status: cancelled.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[github-actions]

Image was built and published successfully. It is available at:

• quay.io/rhdh-community/rhdh:pr-4901
• quay.io/rhdh-community/rhdh:pr-4901-b8506ea2

[github-actions]

This PR is stale because it has been open 7 days with no activity. Remove stale label or comment or this will be closed in 21 days.