Add LCP documentation

[jslee02]

Adds documentation for the LCP problem and solver selection.

• Adds background docs under docs/background/lcp covering the problem statement, pivoting, projection, Newton, and other methods plus a selection guide.
• Adds dart/math/lcp/README.md documenting the lcp API layout.
• Refreshes docs/dev_tasks/README.md and docs/onboarding/constraints.md to point to the new docs.

---

Before creating a pull request

• Run `pixi run test-all` to lint, build, and test your changes (docs-only change)
• Add unit tests for new functionality (docs-only change)
• Document new methods and classes
• Add Python bindings (dartpy) if applicable (not applicable)

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dart/dart/lcpsolver/detail/DeprecatedNamespace.hpp

Lines 6 to 10 in a646db2

	#if !defined(DART_LCPSOLVER_SUPPRESS_DEPRECATED_HEADER_WARNING)
	#if defined(_MSC_VER)
	#pragma message( \
	__FILE__ " is deprecated; include dart/math/lcp equivalents instead")
	#else

Deprecated lcpsolver headers no longer expose old namespace

DeprecatedNamespace.hpp used to provide a deprecated dart::lcpsolver alias to keep pre-existing includes building; now it only emits a warning and ends, so code including dart/lcpsolver/dantzig/lcp.h can no longer refer to dart::lcpsolver::* and will fail to compile. If compatibility for the deprecated namespace is still required, the alias needs to be restored or callers will break immediately after this change.

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Restore dart::math::lcp namespace compatibility

The LCP headers now declare Lemke directly in dart::math and removed the former dart::math::lcp namespace/using alias. Any downstream code that still calls the documented legacy APIs via dart::math::lcp::Lemke/SolveLCP will now fail to compile even though the header path is unchanged. This is a breaking API change disguised as a refactor; if backward compatibility is intended, the lcp namespace alias needs to remain or a compatibility wrapper added.

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