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

bin/gitrev.sh

@@ -1,7 +1,12 @@
-#!/bin/bash
-#
-if [[ ! -z "$GITREV" ]]; then
-    echo $GITREV
-else
-    git describe --tags --always --dirty=-dirty
-fi
+#!/usr/bin/env bash
+set -euo pipefail
+
+main() {
+    if [[ -n "${GITREV:-}" ]]; then
+        echo "$GITREV"
+    else
+        git describe --tags --always --dirty=-dirty
+    fi
+}
+
+main "$@"