docs: target status for 0.2.0
docs: cmpxchg orderings, refusal lists and opt-level spelling
docs: compiler runtime, lamina environment and current target behaviour
docs: s390x lowers inline assembly
docs: environment axis, compiler runtime and current target state
docs: current targets, vectors and nightly gate
docs: describe ir as typed and block-based, not ssa
docs: drop writeptr from windows io note
docs: bring status, targets and ci up to date
docs: fix links dangling after artery split
docs: split artery target onto its own page
docs: link new pages from home and roadmap
docs: add toolchain roadmap