chore: first draft - service as primary approach

[TomCools]

This is the first draft of restructuring the docs so they more align with the service approach.

Solves: #2413 (comment)

Personal TODOs before making this a full PR.

• Validate visually
• Agree on new nav structure
• Validate link backwards compatibility (or deal with broken links decently)
• Clean up wording where it is still inconsistent / biased.
• Add extra tasks for missing items.

[TomCools]

Notes to self after visual validation.

• SolverModel interface, this should probably just move to the core "modelling planning problems" docs instead of specific for Service.
• How library solving works, don't like that wording
• Tuning and diagnostics should move to the new tuning section