docs: update how-to and reference landing pages

[florentianayuwono]

What this PR does

Updates the how-to and reference landing pages to improve documentation quality.

• Removes meta-descriptions from all sections (phrases like "The following guides cover...", "This section contains...", etc.)
• Merges the orphan "Development" section (single page) into "Update and refresh", renaming the combined section "Maintenance and development"
• Merges the orphan "Architecture and deployments" section (single page) into a new "Configuration and operations" section in the reference index
• Adds visible bullet-list links to each section for easier navigation
• Adds meaningful, subject-focused section descriptions that explain context rather than describing the documentation itself

Why we need it

Meta-descriptions add noise without value — readers can see what pages are listed. Orphan sections (single page under a heading) fragment the navigation unnecessarily. These changes follow the documentation standards demonstrated in the wordpress-k8s-operator docs.

Checklist

• I followed the contributing guide
• I added or updated the documentation (if applicable)
• I updated docs/changelog.md with user-relevant changes
• I used AI to assist with preparing this PR
• I added or updated tests as needed (unit and integration)
• 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)
• If this is a Grafana dashboard: I added a screenshot of the dashboard
• If this is Terraform: terraform fmt passes and tflint reports no errors
• If this is Rockcraft: I updated the version

Test plan

Documentation-only change. Verified by reviewing the rendered RST structure: toctree entries are preserved, bullet links are correct, and no orphan sections remain.

Review focus

• Section groupings and naming: does "Maintenance and development" feel right for back-up-restore + upgrade + contribute?
• Section groupings in reference: does "Configuration and operations" (actions, configurations, charm-architecture) + "Connectivity and monitoring" (integrations, metrics) make sense?
• Section descriptions: are they meaningful without being meta?