Describe the 2025-2026 state and structure of the repository

Summary:
Repository structure is slightly evolving.

Also, the repository started as a work-in-progress, and the README
still reflected that. With more than 47k LOC, it's nowadays the bulk
of the Nasqueron configuration that's there.

Proofread grammar and text clarity.

Test Plan: Read again the document

Differential Revision: https://devcentral.nasqueron.org/D3688