docs: fix the credential and external MongoDB examples - #162

Summary

Two example files have three faults. A user who copies them gets a broken installation.

Details

• examples/graylog-secret.yaml: use stringData in place of data. The committed values decoded to 65 and 97 bytes, not 64 and 96. The hash kept a newline from cut, and the pepper kept one from pwgen. The commands in the file made the same fault for every user. All credential values are now placeholders, and the hash command uses printf. GRAYLOG_ROOT_USERNAME keeps the value admin, and the two MaxMind keys stay empty.
• examples/values-existing-secret-external-mongodb.yaml: delete the second graylog: key. YAML keeps the last duplicate, so the file discarded its own replicas: 3 and its resource limits.
• Same file: do not send GRAYLOG_MONGODB_URI through base64 before --from-literal. kubectl encodes the value, so Graylog got base64 text in place of a URI. The init container then stopped.
• docs/bring-your-own-mongo.md: the text said that the example uses base64 encoded variables. This is not correct.

Linked issues

None.

PR Checklist

Please check the items that apply to your change.

• Tests added/updated
• Documentation updated
• This PR includes a new feature
• This PR includes a bugfix
• This PR includes a refactor

Testing Checklist

Static Validation

• Linter check passes: helm lint ./charts/graylog
• Helm renders local template sucessfully: helm template graylog ./charts/graylog --validate

--validate needs the MongoDB Operator CRD. The test cluster does not have it. helm template without --validate passes.

Installation

• Fresh installation completes successfully: helm install graylog ./charts/graylog
• All pods reach Running state: kubectl rollout status statefulset/graylog
• Helm tests pass: helm test graylog

Functional (if applicable)

• Web UI accessible and login works
• DataNodes visible in System > Cluster Configuration
• Inputs can be created and receive data

Upgrade (if applicable)

• Upgrade from previous release succeeds
• Scaling up/down works correctly
• Configuration changes apply correctly

This change edits two examples and one document. The installation, functional, and upgrade tests did not run.

Specific to this PR

• Rendered the external MongoDB example before and after. Before: replicas=2 and a memory limit of 2Gi, which are the chart defaults. After: replicas=3 and a memory limit of 3Gi, which the file asks for.
• kubectl apply --dry-run=client -f examples/graylog-secret.yaml passes.
• The new hash command returns a digest of 64 characters. cut still adds a newline, so the command writes 65 bytes. The digest that a reader copies is 64 characters.
• Both example files parse as YAML. helm unittest passes: 332 tests in 28 suites.

Notes for reviewers

• Verify all applicable tests above pass
• Validate that the linked issues are no longer reproducible, if applicable
• Sync up with the author before merging
• The commit history should be preserved - use rebase-merge or standard merge options when applicable