[libc][docs][NFC] Restructure Getting Started guide and update Build Concepts. (#187701)

Restructured the Getting Started guide into a numbered step-by-step path
for easier readability. Added a Hello World verification step to confirm
build integrity after build completion.

Additionally, updated build_concepts.rst and the Getting Started guide
to clarify that Overlay Mode is intended for augmenting the system's C
library rather than incremental adoption.

NOKEYCHECK=True
GitOrigin-RevId: 2b47497f6e0a3d11443e55d6ada333c7883833e5