Document updates to QBFT and IBFT configurations

[rolandtyler]

Pull request checklist

Use the following template to make sure your PR fits the GoQuorum documentation standard.

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Make sure that:

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Describe the change

Updates to QBFT and IBFT configurations:

• Updated genesis files with ibft and qbft attributes
• Document smart contract validators
• document transitions
• remove deprecated CLI options --istanbul.requesttimeout and --istanbul.blockperiod

Issue fixed

Fixes #218
Fixes #230
Fixes #231

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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[antonydenyer]

Nice.

[bgravenorst]

Minor feedback, otherwise LGTM