docs: rewrite the README in the project tone

Rewrites the README for flow rather than structure. No em-dashes linking
thoughts, no colons introducing explanations in prose, no semicolons, and
contractions throughout. Sentences vary in length so the rhythm reads as
written prose instead of a list of assertions.

Each section now establishes why the problem matters before explaining the
mechanism. The opening leads with what running a model on AMD normally costs
you in layers, the collectives section leads with the fact that decode hits a
collective every token, and the attention section leads with what actually
sets tokens per second at long context.

Every measurement, command, and non-claim from the previous revision is
preserved verbatim.

Author: maincode-dave