docs: add backends and targets content pages

[nceglia]

• docs/source/backends.rst — MI computation pipeline (discretization, joint histogram, MI formula, signing), backend dispatch table with speedups, selection examples, runtime benchmarks, and architecture overview
• docs/source/targets.rst — Built-in targets table, usage examples, graph-aware targets with spatial/TCR examples, custom target registration, and caching details

Sphinx builds cleanly with no structural errors.