docs(config): clarify environment variable loading in Vite config

[abdelrhman-arnos]

Description

Clarifies how and when .env files are loaded relative to config evaluation.

Fixes #20611.

Changes limited to:

• docs/config/index.md: Replaces ambiguous wording that implied Vite doesn't load .env at all with an explanation that loading is deferred until after config resolution, and adds guidance on when to use loadEnv.

No other files touched, Env & Mode page intentionally left unchanged (it was already accurate).

[sapphi-red]

Thanks!