Add information about emptyBlockPeriodSeconds

[antonydenyer]

Describe the change

Add basic documentation about how to configure empty block period implemented in Consensys/quorum#1433

Impacted parts

For content changes:

• Documentation content
• Documentation page organization

For tool changes:

• Build and QA tools (e.g., lint)
• MkDocs templates
• MkDocs configuration
• Python dependencies
• Node dependencies and JavaScript
• GitHub integration

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• You've fixed any issues raised by the tests.
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[rolandtyler]

Minor edits. I also moved the section up so it was just after setting the blockPeriodSeconds since they're related. LGTM