[docs] Add VirtualDisk metadata.name length limit

[prismagod]

Description

Added documentation about the maximum length of VirtualDisk.metadata.name (60 characters).

Why do we need it, and what problem does it solve?

Previously, this constraint was not explicitly documented.

What is the expected result?

Checklist

• The code is covered by unit tests.
• e2e tests passed.
• Documentation updated according to the changes.
• Changes were tested in the Kubernetes cluster manually.

Changelog entries

section: docs
type: docs
summary: "Added documentation about the maximum length of VirtualDisk.metadata.name."
impact_level: low