Document the Nix Archive format

This is adopted from Eelco's PhD thesis.

doc/manual/src/SUMMARY.md.in

@@ -110,6 +110,7 @@
     - [Derivation](protocols/json/derivation.md)
   - [Serving Tarball Flakes](protocols/tarball-fetcher.md)
   - [Store Path Specification](protocols/store-path.md)
+  - [Nix Archive (NAR) Format](protocols/nix-archive.md)
   - [Derivation "ATerm" file format](protocols/derivation-aterm.md)
 - [Glossary](glossary.md)
 - [Contributing](contributing/index.md)

doc/manual/src/protocols/nix-archive.md

@@ -0,0 +1,42 @@
+# Nix Archive (NAR) format
+
+This is the complete specification of the Nix Archive format.
+The Nix Archive format closely follows the abstract specification of a [file system object] tree,
+because it is designed to serialize exactly that data structure.
+
+[file system object]: @docroot@/store/file-system-object.md
+
+The format of this specification is close to [Extended Backus–Naur form](https://en.wikipedia.org/wiki/Extended_Backus%E2%80%93Naur_form), with the exception of the `str(..)` function / parameterized rule, which length-prefixes and pads strings.
+This makes the resulting binary format easier to parse.
+
+Regular users do *not* need to know this information.
+But for those interested in exactly how Nix works, e.g. if they are reimplementing it, this information can be useful.
+
+```ebnf
+nar = str("nix-archive-1"), nar-obj;
+
+nar-obj = str("("), nar-obj-inner, str(")");
+
+nar-obj-inner
+  = str("type"), str("regular") regular
+  | str("type"), str("symlink") symlink
+  | str("type"), str("directory") directory
+  ;
+
+regular = [ str("executable"), str("") ], str("contents"), str(contents);
+
+symlink = str("target"), str(target);
+
+(* side condition: directory entries must be ordered by their names *)
+directory = str("type"), str("directory") { directory-entry };
+
+directory-entry = str("entry"), str("("), str("name"), str(name), str("node"), nar-obj, str(")");
+```
+
+The `str` function / parameterized rule is defined as follows:
+
+- `str(s)` = `int(|s|), pad(s);`
+
+- `int(n)` = the 64-bit little endian representation of the number `n`
+
+- `pad(s)` = the byte sequence `s`, padded with 0s to a multiple of 8 byte