Reduce the root README to project discovery and navigation: what
irtt-rs is, its relationship to upstream IRTT (including a factual
capability comparison checked against upstream v0.9.2), installation,
a short quick start, and links out to detailed documentation. Detailed
CLI behavior, output/measurement semantics, continuous-mode memory
guidance, and server bind/session/negotiation semantics move into new
manpage-shaped docs under docs/ (irtt-rs, irtt-client, irtt-tui,
irtt-server), rewritten as operational guidance rather than copied
verbatim from the old README sections they replace.
Nils Andreas Svee