[codex] fix documentation page paths

[jayren3996]

Summary

• replace stale root docs pages with pointers to the maintained Documenter sources
• make the docs manifest use the repository-relative MPSToolkit path
• add a regression test that catches machine-local /Users/... paths in user-facing docs inputs

Root cause

The repository had duplicate root docs pages with stale absolute local paths, and the docs manifest still pointed at an old local checkout path.

Validation

• julia --color=yes --project=docs docs/make.jl
• julia --color=yes --project=. -e 'using Test; include("test/test_docs.jl")'\n- julia --color=yes --project=. test/runtests.jl