Move FORMAT.md to docs/ and contrib/ to bin/, fix bash script style (closes #74)

FORMAT.md moves to docs/ and every reference follows; its two relative
links to mfer/mf.proto now point up one directory, its text is otherwise
unchanged. contrib/usage.sh moves to bin/, not docs/: it is a script you
run (it builds mfer, then generates and checks a manifest of the repo),
not reading material. Both scripts now use #!/usr/bin/env bash,
set -euo pipefail and a main function. bin/gitrev.sh still exits non-zero
outside a git checkout, so the Makefile still falls back to unknown.
.gitignore now ignores only the built bin/mfer rather than all of bin/,
which holds tracked scripts.

Model: opus-5-5

README.md

@@ -242,7 +242,7 @@ are now tracked only in the [issues](https://git.eeqj.de/sneak/mfer/issues).
   as question 10 on [issue 82](https://git.eeqj.de/sneak/mfer/issues/82).
 
 - Should the on-disk serialization format be proto3 or json? Settled: it is
-  proto3, see `FORMAT.md` and `mfer/mf.proto`.
+  proto3, see `docs/FORMAT.md` and `mfer/mf.proto`.
 
 # Tool Examples