[BOP-1572] Add etcd configuration and custom flag migration info

[nsteph]

This change adds:

• etcd configuration page (similar to other Kube components)
• subsections on the MKE 3 configuration migration docs that cover kubernetes custom flags and kubelet custom flag profiles

[KoryKessel-Mirantis]

Suggested change

	MKE 3 configuration file:
	```
	[cluster_config.custom_kube_api_server_flags] = ["--enable-garbage-collector=false"]
	```
	MKE 4 configuration file:
	```
	spec:
	apiServer:
	extraArgs:
	enable-garbage-collector: false
	```
	- MKE 3 configuration file:

[cluster_config.custom_kube_api_server_flags] = ["--enable-garbage-collector=false"]

- MKE 4 configuration file:

spec:
apiServer:
extraArgs:
enable-garbage-collector: false

[KoryKessel-Mirantis]

Not sure that accepting the suggestion will result in a correct rendering, so I will try to offer here how I would like to see this example present.

• MKE 3 configuration file:

  [cluster_config.custom_kube_api_server_flags] = ["--enable-garbage-collector=false"]
  

• MKE 4 configuration file:

  spec
    apiServer:
      extraArgs:
        enable-garbage-collector: false
  

[nsteph]

I tried to match this by bullet pointing the config examples.

[nsteph]

@KoryKessel-Mirantis As discussed, I based the etcd pag on the existing MKE 3 docs, just changing the text to match the MKE 4 config and featureset.

The other sections (defrag/cleaning/alarm) describe features not yet implemented in MKE 4.