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

.gitignore

@@ -1,4 +1,4 @@
-/bin/
+/bin/mfer
 /tmp
 /node_modules/
 *.tmp