Add stubs for environment variables; document some of the important ones

This uses a very hacky regex that will probably miss some variables. But
having some docs seems better than none at all.

This uses a very hacky regex that will probably miss some variables. But having some docs seems better than none at all.

In particular, this generates stubs for the following env vars:

- COLORTERM
- QNX_TARGET
- RUST_BACKTRACE
- RUSTC_BLESS
- RUSTC_BOOTSTRAP
- RUSTC_BREAK_ON_ICE
- RUSTC_CTFE_BACKTRACE
- RUSTC_FORCE_RUSTC_VERSION
- RUSTC_GRAPHVIZ_FONT
- RUSTC_ICE
- RUSTC_LOG
- RUSTC_OVERRIDE_VERSION_STRING
- RUSTC_RETRY_LINKER_ON_SEGFAULT
- RUSTC_TRANSLATION_NO_DEBUG_ASSERT
- RUST_DEP_GRAPH_FILTER
- RUST_DEP_GRAPH
- RUST_FORBID_DEP_GRAPH_EDGE
- RUST_MIN_STACK
- RUST_TARGET_PATH
- SDKROOT
- TERM
- UNSTABLE_RUSTDOC_TEST_LINE
- UNSTABLE_RUSTDOC_TEST_PATH

[rendered](![screenshot of unstable-book running locally, with 14 environment variables shown in the sidebar](https://github.com/user-attachments/assets/8238d094-fb7a-456f-ad43-7c07aa2c44dd))

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