Add IronCore and metal-stack docs

[BoHristova]

How to categorize this PR?

/kind enhancement

What this PR does / why we need it:

The IronCore and metal-stack extensions are mentioned on the landing page, but their docs aren't part of the List of Extensions section.

Landing page:

List of Extensions | Infrastructure Extensions:

What was done:

• Added subsections for the IronCore and metal-stack extensions to Documentation | List of Extensions.
• Added a subsection for the IronCore extension to Contribute | Extensions.
• For the IronCore extension: Added the docs via Docforge, since the doc/ directory in the repo follows the persona-based structure.
• For the metal-stack subsection in the main documentation: Added an index page that points readers to the GitHub repository, since no dedicated docs exist (see README).

Which issue(s) this PR fixes:

Fixes #1010
Related to #1023

Special notes for your reviewer:

Summary by CodeRabbit

• Documentation
  • Added documentation for the IronCore infrastructure provider extension, including development resources.
  • Added documentation infrastructure entries for the IronCore and Metal provider extensions.
  • Added an overview page for the Metal Stack provider, including maintenance details and repository links.

[gardener-prow]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[coderabbitai]

📝 Walkthrough

Walkthrough

Adds IronCore and metal-stack to the infrastructure extension documentation structure. The changes register IronCore contribution sources through Docforge and create a metal-stack Hugo page linking to its repository.

Changes

Infrastructure extension documentation

Layer / File(s)	Summary
Provider documentation registration .docforge/documentation/gardener-extensions/infrastructure.yaml, hugo/content/docs/extensions/infrastructure-extensions/gardener-extension-provider-metal/index.md	Registers IronCore and metal-stack in the infrastructure extension structure and adds metadata and descriptive content for the metal-stack page.
IronCore contribution documentation .docforge/contribute.yaml	Configures the IronCore README and development documentation sources for contribution documentation.

Estimated code review effort: 1 (Trivial) | ~5 minutes

Possibly related issues

• gardener/documentation#1023 — Covers adding IronCore and metal-stack documentation entries through Docforge and Hugo.

Possibly related PRs

• gardener/documentation#938 — Also extends the infrastructure provider structure in .docforge/documentation/gardener-extensions/infrastructure.yaml.
• gardener/documentation#1006 — Modifies the same infrastructure provider structure while removing an older provider entry.

Suggested reviewers: klocke-io, n-boshnakov

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	The changes satisfy issue #1010 by adding IronCore and metal-stack docs, the IronCore contribute entry, and the metal-stack index page.
Out of Scope Changes check	✅ Passed	No clearly unrelated or out-of-scope changes are evident beyond the requested documentation updates.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Title check	✅ Passed	The title clearly matches the main change: adding IronCore and metal-stack documentation.
Description check	✅ Passed	The description follows the template and includes kind, purpose, fixes, and reviewer notes, with only minor detail missing.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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