Add missing .md to the MetalBox ZTP link

[ideaship]

The link to the SONiC ZTP section omits the file extension:

[MetalBox and rolled out to the switches via ZTP](./metalbox#sonic-ztp)

Docusaurus resolves extensionless links, so the site builds and the link works.
lychee does not: it checks the literal path docs/concepts/metalbox, which does
not exist on disk, and reports File not found on every run. That report is
non-blocking, so it has been sitting in the MegaLinter output as a permanent
error rather than failing anything.

This is a deviation rather than a broken link. Explicit .md is the convention
here — 100 internal links use it, including the very next link in this same file
(./cluster-network.md) — and this was the only extensionless file link left in
docs/. Adding the extension changes no rendered output.

Verification

• docs/concepts/metalbox.md exists, so the path lychee checks now resolves.
• The anchor is real: ## SONiC ZTP at metalbox.md:161, which Docusaurus
  slugifies to #sonic-ztp.
• markdownlint and markdown-table-formatter clean.

I could not run lychee locally (no binary available; it is a Rust tool rather
than an npm package), so the expectation that this clears the warning is
inferred from lychee's own error message — the literal path it reported as
missing now exists. The MegaLinter run on this PR is the actual confirmation.

Noticed while looking at an unrelated MegaLinter failure on
#1048; this link is pre-existing and independent of that
change.

🤖 Generated with Claude Code

[github-actions]

✅⚠️MegaLinter analysis: Success with warnings

Descriptor	Linter	Files	Fixed	Errors	Warnings	Elapsed time
✅ ACTION	actionlint	5		0	0	0.03s
✅ JSON	jsonlint	4		0	0	0.08s
✅ JSON	prettier	4		0	0	0.36s
✅ JSON	v8r	4		0	0	10.56s
✅ MARKDOWN	markdownlint	158		0	0	2.46s
✅ MARKDOWN	markdown-table-formatter	158		0	0	0.44s
✅ REPOSITORY	betterleaks	yes		no	no	0.88s
✅ REPOSITORY	checkov	yes		no	no	21.21s
✅ REPOSITORY	git_diff	yes		no	no	0.08s
✅ REPOSITORY	secretlint	yes		no	no	1.94s
✅ REPOSITORY	trufflehog	yes		no	no	4.5s
✅ SPELL	codespell	168		0	0	0.61s
⚠️ SPELL	lychee	168		1	0	27.31s
✅ YAML	prettier	6		0	0	0.46s
✅ YAML	v8r	6		0	0	7.68s
✅ YAML	yamllint	6		0	0	0.63s

Detailed Issues

⚠️ SPELL / lychee - 1 error

📝 Summary
---------------------
🔍 Total..........897
🔗 Unique.........726
✅ Successful.....841
⏳ Timeouts.........4
🔀 Redirected.......3
👻 Excluded........52
❓ Unknown..........0
🚫 Errors...........0
⛔ Unsupported......0

Errors in docs/guides/deploy-guide/services/openstack.md
[TIMEOUT] https://www.openstack.org/software/project-navigator/openstack-components#openstack-services (at 14:5) | Request timed out

Errors in docs/guides/user-guide/openstack/migration-vmware-esxi.md
[TIMEOUT] https://www.openstack.org/vmware-migration-to-openstack/ (at 20:1) | Request timed out

Errors in docs/release-notes/osism-7.md
[TIMEOUT] https://www.openstack.org/software/openstack-bobcat (at 978:38) | Request timed out

Errors in docs/release-notes/osism-8.md
[TIMEOUT] https://www.openstack.org/software/openstack-caracal (at 223:38) | Request timed out

Hint: Followed 3 redirects. You might want to consider replacing redirecting URLs with the resolved URLs. Use verbose mode (`-v`/`-vv`) to see redirection details.

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