feat: Add support for custom guidance and appendices in HTML reports

[leonamagaya]

What does this PR do?

This PR adds support for customizable guidance and appendices content in Cloudsplaining HTML reports. Users can now provide their own HTML content files to customize the guidance and appendices sections of their reports.

Key features:

• Custom content files: Users can create custom-guidance.html and custom-appendices.html files in their working directory
• Automatic detection: The system automatically detects and loads custom content files when present
• Fallback behavior: Falls back to default content when custom files don't exist
• Navigation control: Empty custom files hide the corresponding navigation sections entirely
• Security: Proper HTML escaping prevents JavaScript injection vulnerabilities
• Flexible usage: Supports mixed scenarios (one custom file, one default)

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Completion checklist

• Additions and changes have unit tests
• The pull request has been appropriately labeled using the provided PR labels
• GitHub actions automation is passing (make test, make lint, make security-test, make test-js)
• If the UI contents or JavaScript files have been modified, generate a new example report:

[Copilot]

Pull Request Overview

This PR adds support for custom guidance and appendices content in Cloudsplaining HTML reports by allowing users to provide custom HTML files that override default content sections.

• Adds TemplateConfig class to detect and process custom HTML files (custom-guidance.html and custom-appendices.html)
• Implements conditional navigation visibility based on content availability
• Integrates custom content rendering in Vue.js components with proper HTML escaping

Reviewed Changes

Copilot reviewed 8 out of 11 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
test/shared/test_template_config.py	Comprehensive test suite covering all template configuration scenarios
cloudsplaining/shared/template_config.py	Core logic for detecting, loading, and processing custom content files
cloudsplaining/output/template.html	JavaScript variable definitions for template configuration
cloudsplaining/output/src/views/Guidance.vue	Vue component updated to render custom or default guidance content
cloudsplaining/output/src/views/Appendices.vue	Vue component updated to render custom or default appendices content
cloudsplaining/output/src/routes/routes.js	Dynamic route registration based on navigation visibility flags
cloudsplaining/output/src/App.vue	Navigation menu updates with conditional visibility and code formatting
cloudsplaining/output/report.py	Integration of TemplateConfig into the report generation process

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