Add missing extension documentation

[BoHristova]

Why is this needed

On the website, there are two different places listing Gardener extensions - the website section List of Extensions and the section Known Extension Implementations.

The two lists are not aligned:

• The website section List of Extensions is missing several actively-maintained extensions.
• The page section Known Extension Implementations is missing the STACKIT and Kubernetes Auditing extensions, which are already in the website docs.
• Both lists are missing several extensions maintained by the metal-stack team.

Extensions to add to the website docs

The following extensions are are missing from List of Extensions:

Extension	Target section	Link
IronCore	Infrastructure Extensions	https://github.com/ironcore-dev/gardener-extension-provider-ironcore
KubeVirt	Infrastructure Extensions	https://github.com/gardener/gardener-extension-provider-kubevirt
metal-stack	Infrastructure Extensions	https://github.com/metal-stack/gardener-extension-provider-metal
vSphere	Infrastructure Extensions	https://github.com/gardener/gardener-extension-provider-vsphere
Debian/Ubuntu (metal-stack)	Operating System Extensions	https://github.com/metal-stack/os-metal-extension
Cloudflare	Others	https://github.com/schrodit/gardener-extension-provider-dns-cloudflare
Audit (metal-stack/gardener-extension-audit)	Others	https://github.com/metal-stack/gardener-extension-audit
gardener-extension-csi-driver-lvm	Others	https://github.com/metal-stack/gardener-extension-csi-driver-lvm
Gardener Extension for S3 Compatible Storage	Others	https://github.com/metal-stack/gardener-extension-backup-s3
Image Rewriter	Others	https://github.com/gardener/gardener-extension-image-rewriter
OIDC Apps Controller	Others	https://github.com/gardener/oidc-apps-controller
PowerDNS	Others	https://github.com/metal-stack/gardener-extension-dns-powerdns
Shoot Falco Service	Others	https://github.com/gardener/gardener-extension-shoot-falco-service
Shoot Flux Service	Others	https://github.com/stackitcloud/gardener-extension-shoot-flux

Extensions to add to Known Extension Implementations

The following extensions are missing from the Known Extension Implementations. They should be added via a separate PR to the gardener/gardener repo:

Extension	Target section	Link
STACKIT	Infrastructure Provider	https://github.com/stackitcloud/gardener-extension-provider-stackit
Auditing (gardener/gardener-extension-auditing)	Generic Extensions / Others	https://github.com/gardener/gardener-extension-auditing
Audit (metal-stack/gardener-extension-audit)	Generic Extensions / Others	https://github.com/metal-stack/gardener-extension-audit
csi-driver-lvm	Generic Extensions / Others	https://github.com/metal-stack/gardener-extension-csi-driver-lvm
Gardener Extension for S3 Compatible Storage	Generic Extensions / Others	https://github.com/metal-stack/gardener-extension-backup-s3

Out of scope

The following extensions are not actively maintained and will not be added to the website docs (however, they will remain listed in Known Extension Implementations):

• Hetzner Cloud
• RFC2136
• k3os
• Kata Containers
• Minimal Working Example

Open questions

1. Extension categories:

• The subcategories in the two lists are currently not the same.
• We need to decide what categories we want to have and align the structures of List of Extensions and Known Extension Implementations accordingly.
• The target sections listed above reflect the current structure of the two lists. Once the structure has been aligned, the placement of extensions currently assigned to Others may change.

2. Pulling content or creating an index page

• Several extensions in the lists above have minimal documentation — placeholder READMEs with little more than a short description and a link to the upstream Gardener docs (see gardener-extension-provider-metal, gardener-extension-dns-powerdns, gardener-extension-backup-s3, os-metal-extension).
• We need to decide how to handle these: Should the content be pulled via Docforge (using the README as the _index.md source), or should we create an index page in the website repo, similarly to the existing ACL extension page?

Definition of Done

• Decide on extension categories and align the structures of the two lists.
• Decide whether extensions without dedicated documentation should have their README pulled via Docforge, or whether an index page should be created in the website repo (as done for the ACL extension).
• Extend the List of Extensions section.
• Extend the Known Extension Implementations list.

[BoHristova]

IronCore API Metal is missing from the list above and needs to be added to both the website section and the list in gardener/gardener.

[BoHristova]

Traefik Ingress is missing from the list of known implementations in gardener/gardener.