This adds several changes to how environment variables are handled to
more closely align with how configs are handled, and to fix an issue
with replacing entire tables. The changes are:
- Top-level tables like `MDBOOK_BOOK` now *replace* the contents of the
`book` table instead of merging it. This adds consistency with how all
the other environment objects work.
- Fixed allowing top-level replacement of `MDBOOK_BOOK` and
`MDBOOK_OUTPUT`. This was inadvertently recently broken.
- Added ability to replace top-level `MDBOOK_RUST`. I don't recall why
that wasn't included.
- Reject invalid keys like `MDBOOK_FOO`.
- Reject unknown keys, like `MDBOOK_BOOK='{"xyz": 123}'`
- Reject invalid types, like `MDBOOK_BOOK='{"title": 123}'`
restructure guide configuration section
restructure guide configuration section
redirect to new config top level
fix broken links
use a relative path for redirect
Co-authored-by: Eric Huss <eric@huss.org>
remove extra heading
