docs: fix broken links and link every guide from a README

[aaronmgn]

Summary

Four repository links are broken, five guides have no inbound link, and one code block mixes a command with its output.

Details

• README.md: [examples](/examples/) used a site-root path, which does not resolve on GitHub. It is now examples/.
• charts/graylog/README.md: the cert-manager section linked to #bring-your-own-certificate-ingress-controller-recommended. The heading is ### Option 1: Bring Your Own Certificate with Ingress Controller (recommended), so the anchor is #option-1-bring-your-own-certificate-with-ingress-controller-recommended.
• docs/TESTING.md: the links to CONTRIBUTING.md and README.md were sibling links. They resolved to docs/CONTRIBUTING.md and docs/README.md, and neither file exists. They now point to ../CONTRIBUTING.md and ../charts/graylog/README.md, which is the chart reference that the link text names.
• README.md: added a Guides section. No document linked to docs/GeoIP_Sidecar_Deployment_Guide.md. Four more pages had no link from either README, but CONTRIBUTING.md:21,32,72 links the Testing, MicroK8s, and Releasing guides, and docs/bring-your-own-opensearch.md:19 links the MongoDB guide.
• docs/graylog-secrets.md: the hash example put a $ prompt and the output in one copyable block. The command is now its own block, and the output is separate. The command uses printf and cut, so it returns the digest that the document prints.

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 documentation only. The installation, functional, and upgrade tests did not run.

Specific to this PR

• Ran a local link checker over every Markdown file. It resolves relative paths and GitHub heading anchors. Before: 4 broken links. After: 0.
• Every page under docs/ now has one inbound link or more from a README. Before this change, no document linked to GeoIP_Sidecar_Deployment_Guide.md.
• printf %s 'my-password' | sha256sum | cut -d ' ' -f1 returns the digest that the document prints.

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

The link checker is not in CI. With it in CI, these four links cannot come back.

[CLAassistant]

All committers have signed the CLA.