CCM-10850: add sidebar section ordering

[afernandez-nhs]

• Introduced a new YAML file to define the order of sections in the sidebar navigation.
• Updated the page layout to utilize the defined section order, ensuring sections not listed appear at the end in alphabetical order.

Description

This change implements a configurable sidebar section ordering system for the NHS Notify documentation. A new YAML configuration file (docs/_data/sidebar-section-order.yml) has been created to define the preferred order of navigation sections. The page layout template has been updated to read this configuration and display sections in the specified order, with any sections not explicitly listed appearing at the end in alphabetical order.

Context

The sidebar navigation was previously displaying sections in an arbitrary order, which could be confusing for users trying to navigate through the documentation. This change provides a structured and predictable navigation experience by allowing explicit control over section ordering while maintaining flexibility for new sections.

Type of changes

• Refactoring (non-breaking change)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would change existing functionality)
• Bug fix (non-breaking change which fixes an issue)

Checklist

• I am familiar with the contributing guidelines
• I have followed the code style of the project
• I have added tests to cover my changes
• I have updated the documentation accordingly
• This PR is a result of pair or mob programming

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Sensitive Information Declaration

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• I confirm that neither PII/PID nor sensitive data are included in this PR and the codebase changes.