fix: move token cache info and delete file
Description
This change reorganizes documentation for the GitLab CLI's token cache feature.
The main documentation file was moved from token-cache.md to _index.md, and detailed explanations about how the cache works were added to both the documentation and the command's help text.
The additions explain that tokens are stored in two secure locations (system keyring and encrypted files) to improve performance when working with Kubernetes clusters, and describe how tokens are organized to keep different GitLab instances separate from each other.