Update architecture page (16)

[a-velasco]

Issue

The Architecture explanation page was out of date (see #1083) and in general need of some polishing.

Solution

• Rewrote the architecture overview for clarity, added notes about snap channels, and clarified the list of shipped components and snap services.
• Polished integration section descriptions for integration charms, added references to related docs.
• Unrelated change: set default syntax highlighting language and style in Sphinx configuration (conf.py) to improve code block rendering in documentation.
• TODO: Address "TODO" blocks in architecture.md - especially the juju ssh sample output on L22, and ldap-sync service on L55.

Checklist

• I have added or updated any relevant documentation.
• I have cleaned any remaining cloud resources from my accounts.