docs: replace more showcase references with RHDH

[christoph-jerolimov]

Description

I updated just some other doc titles and references from showcase to RHDH.

Which issue(s) does this PR fix

• Part of https://issues.redhat.com/browse/RHIDP-5574

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

[github-actions]

The image is available at:

• quay.io/rhdh-community/rhdh:pr-2241 or
• quay.io/rhdh-community/rhdh:pr-2241-9c7533f3

[dzemanov]

/lgtm

[openshift-ci]

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