Fix unescaped angle brackets in vSphere extension docs and add extension to website

[BoHristova]

Summary

The documentation for the vSphere provider extension contains unescaped angle brackets: https://github.com/gardener-attic/gardener-extension-provider-vsphere/blob/642bbe1c04fb7e2f9f767cad5b87c42b16057af8/docs/usage/prepare-vsphere.md?plain=1#L237.

This causes a build error when being pulled into the public website (see #1068 (comment)).

What needs to be done

• Open a PR in Wrap the angle brackets https://github.com/gardener-attic/gardener-extension-provider-vsphere/ to wrap the angle brackets in backticks.
• Add the extension to the Extensions section of the website.

[BoHristova]

Closing because the vSphere extension is not actively maintained anymore, and its documentation doesn't have to be added to the Gardener website.