chore(tvix/eval): move eval docs to tvix/docs

Change-Id: I75b33c43456389de6e521b4f0ad46d68bc9e98f6
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11809
Autosubmit: flokli <flokli@flokli.de>
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI

tvix/eval/docs/recursive-attrs.md

@@ -1,68 +0,0 @@
-Recursive attribute sets
-========================
-
-The construction behaviour of recursive attribute sets is very
-specific, and a bit peculiar.
-
-In essence, there are multiple "phases" of scoping that take place
-during attribute set construction:
-
-1. Every inherited value without an explicit source is inherited only
-   from the **outer** scope in which the attribute set is enclosed.
-
-2. A new scope is opened in which all recursive keys are evaluated.
-   This only considers **statically known keys**, attributes can
-   **not** recurse into dynamic keys in `self`!
-
-   For example, this code is invalid in C++ Nix:
-
-   ```
-   nix-repl> rec { ${"a"+""} = 2; b = a * 10; }
-   error: undefined variable 'a' at (string):1:26
-   ```
-
-3. Finally, a third scope is opened in which dynamic keys are
-   evaluated.
-
-This behaviour, while possibly a bit strange and unexpected, actually
-simplifies the implementation of recursive attribute sets in Tvix as
-well.
-
-Essentially, a recursive attribute set like this:
-
-```nix
-rec {
-  inherit a;
-  b = a * 10;
-  ${"c" + ""} = b * 2;
-}
-```
-
-Can be compiled like the following expression:
-
-```nix
-let
-  inherit a;
-in let
-  b = a * 10;
-  in {
-    inherit a b;
-    ${"c" + ""} = b * 2;
-  }
-```
-
-Completely deferring the resolution of recursive identifiers to the
-existing handling of recursive scopes (i.e. deferred access) in let
-bindings.
-
-In practice, we can further specialise this and compile each scope
-directly into the form expected by `OpAttrs` (that is, leaving
-attribute names on the stack) before each value's position.
-
-C++ Nix's Implementation
-------------------------
-
-* [`ExprAttrs`](https://github.com/NixOS/nix/blob/2097c30b08af19a9b42705fbc07463bea60dfb5b/src/libexpr/nixexpr.hh#L241-L268)
-  (AST representation of attribute sets)
-* [`ExprAttrs::eval`](https://github.com/NixOS/nix/blob/075bf6e5565aff9fba0ea02f3333c82adf4dccee/src/libexpr/eval.cc#L1333-L1414)
-* [`addAttr`](https://github.com/NixOS/nix/blob/master/src/libexpr/parser.y#L98-L156) (`ExprAttrs` construction in the parser)