pw_env_setup: Add section headers to docs
pw_env_setup: Add section headers to docs

I was skimming the docs earlier today and skipped the entire
"Environment Variables" section after reading the first sentence,
because it suggested they're all "input" variables. The
section also discusses "output" variables, but it was not clear where
they started. Adding some section headers to make this easier to
navigate.

Open to using different terminology than "input" and "output".

Change-Id: I9e4767d402a6f8a491fc2a093bee78d4626524bf
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/88260
Reviewed-by: Rob Mohr <d07614a8a0b466db08f68224f87d738b0f9615cf@google.com>
Commit-Queue: Ted Pudlik <c7372cc3e451acc046049fca9e050bcb2a4a3d11@google.com>

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