docs: clarify env names are configurable, roles are positional

[joshua-temple]

Problem

The docs use dev/test/uat/prod throughout, which can read as reserved names. Nothing in the generator treats those labels specially, and the env roles are decided by position, not by name. Readers had no single statement of that.

Fix

Three surgical, documentation-only edits:

• docs/src/content/docs/configuration.md (authoritative): a note aside in the environments area stating names are fully configurable, roles are positional (last = release stage, second-to-last = prerelease), the example names are illustrative, the env count is structural, plus naming guidance (identifier-safe names, reserved environment/dry_run dispatch inputs, gha_environment maps to a real GitHub Environment).
• README.md: one line at the promote modes table noting the modes are generated from your configured env names and roles are positional.
• docs/src/content/docs/workflows.md: one clause in Promotion Modes noting the env names and <from>-to-<to> modes come from your configuration, with positional roles.

Mermaid diagrams untouched; no per-mention annotations added.

Verification

• Docs build passes (astro build, 9 pages, search index built).
• Guardrail scan on the diff: zero forbidden strings, zero em dashes, no process language.