docs: clarify server_url on cluster creation (#1002)

* docs: clarify server_url on cluster creation

the creating node may not have a server_url set
this is due to harvester-installer config validator
the config-valudator needs to do this thus harvester checks the value

see https://github.com/harvester/harvester-installer/blob/8317a66fd50b7954f8d1a87a957abbb11994bfbc/pkg/console/validator.go\#L422
see https://github.com/harvester/harvester-installer/blob/8317a66fd50b7954f8d1a87a957abbb11994bfbc/pkg/config/cos.go\#L377

fixes harvester/harvester#10325

Signed-off-by: Lukas Paluch <paluch@tuxig.com>

* Update docs/install/harvester-configuration.md

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
Signed-off-by: Lukas f. Paluch <paluch@tuxig.com>

---------

Signed-off-by: Lukas Paluch <paluch@tuxig.com>
Signed-off-by: Lukas f. Paluch <paluch@tuxig.com>
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Author: fluktuid

docs/install/harvester-configuration.md

@@ -21,7 +21,7 @@ Harvester configuration file can be provided during manual or automatic installa
 
 ```yaml
 scheme_version: 1
-server_url: https://cluster-VIP:443
+server_url: "" # empty or absent in create mode; set to https://cluster-VIP:443 in join mode
 token: TOKEN_VALUE
 os:
   ssh_authorized_keys:
@@ -146,6 +146,7 @@ Make sure that your custom configuration always has the correct scheme version.
 `server_url` is the URL of the Harvester cluster, which is used for the new `node` to join the cluster.
 
 This configuration is mandatory when the installation is in `JOIN` mode. The default format of `server_url` is `https://cluster-VIP:443`.
+In `CREATE` mode the parameter must be absent or with empty value (`""`). Otherwise the installation fails.
 
 :::note
 
@@ -155,6 +156,16 @@ To ensure a high availability (HA) Harvester cluster, please use either the Harv
 
 #### Example
 
+for cluster creation
+
+```yaml
+server_url: '' # or remove this line
+install:
+  mode: create
+```
+
+or to join a cluster
+
 ```yaml
 server_url: https://cluster-VIP:443
 install: