Generate CLI environment variables reference

[chaptersix]

Summary

• generate an environment-variables.mdx page from options with implied-env
• include variable name, related flag, description, and default value
• add a gen-docs test check for the new page
• adapt the implementation to the current split-doc generator

Supersedes #1023, which conflicts with current main.
Fixes #697.

Test plan

• go test ./cmd/gen-docs ./internal/commandsgen
• go test ./...
• generate docs locally and inspect environment-variables.mdx

[chaptersix]

Closing in favor of updating the original PR #1023 directly.