docs: say which file a lowering rule's code goes in #1061

Problem

Step 3 of "Add a lowering rule to the OpenAPI compiler" in CONTRIBUTING.md says the rule's code goes where the rules near it are lowered, and names packages. Once each construct has its own file, the recipe should also say which file.

Direction

After "micro-compiler-design.md says why each package holds what it does." add: "Within the package, the code goes in the file named for its construct (allof.go, responses.go, headers.go) and its tests in that file's _test.go or _internal_test.go; a construct with no file gets one. A reviewer checks this; no test does."

Blocked by

#1057, #1059 and #1060: the sentence describes the layout they produce.

Priority: P4, rule 1 (documentation; changes no behaviour). git grep -n 'file named for its construct' CONTRIBUTING.md → no matches (morphic @a1f15b29).