quorum-board#197: add qlight token manager plugin

[baptiste-b-pegasys]

Pull request checklist

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

Before creating the PR

Make sure that:

• You've read the contribution guidelines.
• You've previewed your changes locally.

Describe the change

Add the documentation for the QLight token management: https://github.com/ConsenSys/quorum-board/issues/197

Issue fixed

https://github.com/ConsenSys/quorum-board/issues/197

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Impacted parts

For content changes:

• Documentation content: qlight node
• 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

After creating your PR and tests have finished

Make sure that:

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

stylistic edits and fixed link. LGTM now.