docs: add the documentation-rewrite specification

[ocots]

The v2.1.0-beta upgrade left docs/ out of scope. The site now documents an API that no longer exists, and the doc environment cannot even resolve against the current package. This adds the specification for the rewrite, as a set of reports under docs/reports/.

Why a specification first: the work is twelve PRs. Fixing the sitemap, the conventions and the per-page contract up front is what lets those PRs be written and reviewed independently rather than as one unreviewable diff.

What the reports establish:

• a sitemap organised by capability (Modelling / Solve / Results / Flows / Geometry / Examples), replacing the single "Manual" node that currently holds thirteen unrelated pages;
• page-by-page content, the exact symbols each page must cover, and what to harvest from the old pages;
• a coverage matrix over the 193 symbols in names(OptimalControl), which is the objective acceptance criterion for the whole effort.

Findings that drive the plan, each verified against the resolved package versions rather than the sibling checkouts:

• the doc environment is broken, not merely stale: docs/Project.toml pins CTBase = "=0.18.8" against 0.28, has neither CTLie nor DifferentiationInterface (without which the whole geometry API is inert), and docs/make.jl:108 fetches a CTFlowsODE extension that was deleted;
• manual-differential-geometry.md is built entirely on Lie, ⋅ and HamiltonianLift, none of which exist;
• manual-macro-free.md:252-279 documents a scalar/vector asymmetry as "intentional" — the "1-D is a scalar" rule removed it, and test/suite/shape/test_shape_contract.jl pins the current behaviour on both the direct and the indirect path;
• api/public.md lists ~10 dead entries and misses every symbol added since;
• flow accessors are asymmetric: control_law and pseudo_hamiltonian are re-exported but hamiltonian and hamiltonian_vector_field are not;
• two capabilities are fully supported and documented nowhere — open/closed loop simulation, and saving/loading solutions.

docs/reports/ sits beside docs/src/, not inside it, so neither Documenter nor VitePress reads it. No behaviour change.