Warn if timeout input parameter value exceeds 10 minutes

[JamesMGreene]

The timeout input parameter is default to 10 minutes:

deploy-pages/action.yml

Lines 12 to 15 in af48cf9

	timeout:
	description: 'Time in milliseconds after which to timeout and cancel the deployment (default: 10 minutes)'
	required: false
	default: '600000'

However, that value is actually more than just a default, it is also effectively a maximum as it also has to contend with the Pages deployment service's backend-enforced timeout of 10 minutes.

We should:

• Input the action.yml description to explain that
• Warn if that input parameter's value exceeds 10 minutes with an explanatory error message
  • This should probably be upgraded to an outright failure of the action step in a future major version bump