docs: Implement new home page pattern and address feedback

[erinecon]

What this PR does

Implements the new home page pattern to the HAProxy charm RTD.

After discussing this PR with Daniele, he had some additional comments on the organization (specifically the load balancing tutorials) and the value behind some of the smaller pages (specifically the upgrade guide). So this PR also addresses those concerns:

• Load balancing tutorials have been reclassified as how-to guides, and some of the content has been refactored to align closer with a how-to guide. The original files were on the edge between a tutorial and a how-to guide (they hold the user's hand through the steps, but lacked explanation to provide the learning experience).
• Upgrade guide has been enhanced to provide more details and reassurance to the user (they don't need to do any backups, and their relations/configurations should remain intact as long as they upgrade along the same channel).

Why we need it

The new home page pattern provides our readers with more clarity and understanding of the domains of concern for the HAProxy charm. Readers now get a high-level overview of the topics and concepts covered in the documentation.

Checklist

• I followed the contributing guide
• I added or updated the documentation (if applicable)
• I updated docs/changelog.md with user-relevant changes
• [N/A] I added a change artifact for user-relevant changes in docs/release-notes/artifacts. If no change artifact is necessary, I tagged the PR with the label no-release-note.
• I used AI to assist with preparing this PR
• [N/A] I added or updated tests as needed (unit and integration)
• [N/A] If integration test modules are used: I updated the workflow configuration
  (e.g., in .github/workflows/integration_tests.yaml, ensure the modules list is correct)
• [N/A] If this PR involves a Grafana dashboard: I added a screenshot of the dashboard
• [N/A] If this PR involves Terraform: terraform fmt passes and tflint reports no errors

I used AI to interrogate the domains and themes in the documentation, using the initial groupings as a starting place. I also used AI to review the changes locally.