Need section for adding etcd extra args

[Tejeev]

https://docs.rke2.io/advanced is missing a section on how to add extra args to etcd.
Related PR and issues:

• [master] Add etcd extra-args support to RKE2 rke2#2136
• [master] Add etcd extra args support for K3s k3s-io/k3s#4463
• Support etcd extra-args rke2#2135

The section should include how to add this in the config as well as as a parameter.

[brandond]

PR welcome.

Note that it is covered in the server args: https://docs.rke2.io/reference/server_config#flags - and functions the exact same as all other flags in terms of how it can be set in the config file. There is no "special" handling for any flag.