Remove unsupported Cloud-in-a-Box guide

[ideaship]

Cloud in a Box is currently unsupported but was prominently featured in the
documentation. This removes it for the time being.

Changes

• Delete the docs/cloud-in-a-box/ guide (index, the virtual-machine
  walkthrough, and its images).
• Drop the Quick start entry that featured it alongside the virtual
  testbed in docs/index.md, and reword the intro since the virtual
  testbed is now the only quick-start path.
• Remove the client-side redirect in docusaurus.config.js whose target no
  longer exists.

The incidental mention of an SCS Cloud-in-a-Box system in the
openstack-health-monitor tool guide is left untouched — it is a factual note
about a CA certificate location rather than a reference to the removed guide.

The removed files remain recoverable from git history should the guide be
reinstated later.

Verification

yarn build passes cleanly with no broken links (onBrokenLinks: 'throw');
the auto-generated sidebar drops the entry automatically.

🤖 Generated with Claude Code

[github-actions]

✅⚠️MegaLinter analysis: Success with warnings

Descriptor	Linter	Files	Fixed	Errors	Warnings	Elapsed time
✅ ACTION	actionlint	5		0	0	0.04s
✅ JSON	jsonlint	4		0	0	0.1s
✅ JSON	prettier	4		0	0	0.38s
✅ JSON	v8r	4		0	0	9.03s
✅ MARKDOWN	markdownlint	154		0	0	2.9s
✅ MARKDOWN	markdown-table-formatter	154		0	0	0.37s
✅ REPOSITORY	betterleaks	yes		no	no	0.62s
✅ REPOSITORY	checkov	yes		no	no	18.65s
✅ REPOSITORY	git_diff	yes		no	no	0.05s
✅ REPOSITORY	secretlint	yes		no	no	1.67s
✅ REPOSITORY	trufflehog	yes		no	no	3.64s
✅ SPELL	codespell	164		0	0	0.58s
⚠️ SPELL	lychee	164		1	0	28.72s
✅ YAML	prettier	6		0	0	0.37s
✅ YAML	v8r	6		0	0	5.71s
✅ YAML	yamllint	6		0	0	0.46s

Detailed Issues

⚠️ SPELL / lychee - 1 error

📝 Summary
---------------------
🔍 Total..........836
🔗 Unique.........686
✅ Successful.....779
⏳ Timeouts........10
🔀 Redirected......49
👻 Excluded........46
❓ Unknown..........0
🚫 Errors...........1
⛔ Unsupported......1

Errors in docs/appendix/security/ossa-2026-001.md
[TIMEOUT] https://bugs.launchpad.net/keystonemiddleware/+bug/2129018 (at 124:3) | Request timed out

Errors in docs/appendix/security/ossa-2026-002.md
[TIMEOUT] https://bugs.launchpad.net/nova/+bug/2137507 (at 111:3) | Request timed out

Errors in docs/appendix/security/ossa-2026-005.md
[TIMEOUT] https://bugs.launchpad.net/keystone/+bug/2142138 (at 109:3) | Request timed out

Errors in docs/appendix/security/ossa-2026-015.md
[TIMEOUT] https://bugs.launchpad.net/keystone/+bug/2148398 (at 174:3) | Request timed out
[TIMEOUT] https://bugs.launchpad.net/keystone/+bug/2148477 (at 175:3) | Request timed out
[TIMEOUT] https://bugs.launchpad.net/keystone/+bug/2149775 (at 176:3) | Request timed out
[TIMEOUT] https://bugs.launchpad.net/keystone/+bug/2149789 (at 177:3) | Request timed out
[TIMEOUT] https://bugs.launchpad.net/keystone/+bug/2150089 (at 178:3) | Request timed out
[TIMEOUT] https://bugs.launchpad.net/keystone/+bug/2150379 (at 179:3) | Request timed out

Errors in docs/release-notes/index.md
[ERROR] https://release.osism.tech/ (at 11:1) | Connection failed. Check network connectivity and firewall settings

Errors in docs/release-notes/osism-9.md
[TIMEOUT] https://bugs.launchpad.net/kolla/+bug/2111620 (at 96:47) | Request timed out

Hint: Followed 49 redirects. You might want to consider replacing redirecting URLs with the resolved URLs. Use verbose mode (`-v`/`-vv`) to see redirection details.

Notices

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