Improve custom ZAPI template instructions

[cgrinds]

See wiki

• Add examples to subtemplates

[vgratian]

This is related to #124

[rajeshl8]

[rajeshl8]

verified build version:
[root@scspa2398581002 harvest]# ./bin/harvest -v
harvest version 21.06.1121-nightly (commit 1524ebb) (build date 2021-06-11T21:11:15+0530) linux/amd64
[root@scspa2398581002 harvest]#

[cgrinds]

Copy screenshot to text to make it easier to copy/paste

validated the document steps and everything worked as expected.
Steps covered:

1. Deploy Harvest and add pollers
2. Now comment all the objects except node object in the conf/zapi/default. yam! file
3. Restart pollers and observe the metrics are generated only for node object
   curl - http://localhost:12991/metrics
4. Now copy the default. yaml to custom.yaml file and uncomment all objects in the custom. yam! file
   except node object
   cp conf/zapi/default.yaml conf/zapi/custom.yaml
5. restart pollers
6. Observe the poller will now collect metrics for all objects
   Note the collector will merge config data from both default and custom. yam| files.