* doc: add stdenv passthru chapter
Broad strokes:
- create the chapter
- move existing stdenv passthru coverage into it
- move out-of-place coverage of passthru.tests from the stdenv meta chapter into it
- (try to) apply 1-sentence-per-line to text I've touched
- add legacy anchors for everything moved
- update existing links to the new anchors
- add tentative motivating text
- make nixpkgs-internal links relative/branchless
razor: if it is only ever needed by contributors, which is likely if links
refer to the latest revision of the source code, then it's for
the contributor guide
Co-authored-by: Valentin Gagarin <valentin.gagarin@tweag.io>
