docs(snix/docs): drop build/index.md

This has already been migrated to //web.

Change-Id: If816a24104ece30ae0826529a9d4df4c4ec3c442
Reviewed-on: https://cl.snix.dev/c/snix/+/30269
Autosubmit: Florian Klink <flokli@flokli.de>
Reviewed-by: Ryan Lahfa <masterancpp@gmail.com>
Tested-by: besadii

snix/docs/src/SUMMARY.md

@@ -28,9 +28,6 @@
 - [Data Model](./castore/data-model.md)
 - [Why not git trees?](./castore/why-not-git-trees.md)
 
-# Builder
-- [Build API](./build/index.md)
-
 # Nix
 - [Specification of the Nix Language](./language-spec.md)
 - [Nix language version history](./lang-version.md)

snix/docs/src/build/index.md

@@ -1,59 +0,0 @@
-# Builder Protocol
-
-The builder protocol is used by tvix-glue to trigger builds.
-
-One goal of the protocol is to not be too tied to the Nix implementation itself,
-allowing it to be used for other builds/workloads in the future.
-
-This means the builder protocol is versatile enough to express the environment a
-Nix build expects, while not being aware of "what any of this means".
-
-For example, it is not aware of how certain environment variables are set in a
-nix build, but allows specifying environment variables that should be set.
-
-It's also not aware of what nix store paths are. Instead, it allows:
-
- - specifying a list of paths expected to be produced during the build
- - specifying a list of castore root nodes to be present in a specified
-   `inputs_dir`.
- - specifying which paths are write-able during build.
-
-In case all specified paths are produced, and the command specified in
-`command_args` succeeds, the build is considered to be successful.
-
-This happens to be sufficient to *also* express how Nix builds works.
-
-Check `build/protos/build.proto` for a detailed description of the individual
-fields, and the tests in `glue/src/snix_build.rs` for some examples.
-
-The following sections describe some aspects of Nix builds, and how this is
-(planned to be) implemented with the Tvix Build protocol.
-
-## Reference scanning
-At the end of a build, Nix does scan a store path for references to other store
-paths (*out of the set of all store paths present during the build*).
-It does do this by (only) looking for a list of nixbase32-encoded hashes in
-filenames (?), symlink targets and blob contents.
-
-While we could do this entirely outside the builder protocol, it'd mean a build
-client would be required to download the produced outputs locally, and do the
-refscan there. This is undesireable, as the builder already has all produced
-outputs locally, and it'd make more sense for it do do it.
-
-Instead, we want to describe reference scanning in a generic fashion.
-
-One proposed way to do this is to add an additional field `refscan_needles` to
-the `BuildRequest` message.
-If this is an non-empty list, all paths in `outputs` are scanned for these.
-
-The `Build` response message would then be extended with an `outputs_needles`
-field, containing the same number of elements as the existing `outputs` field.
-In there, we'd have a list of numbers, indexing into `refscan_needles`
-originally specified.
-
-For Nix, `refscan_needles` would be populated with the nixbase32 hash parts of
-every input store path and output store path. The latter is necessary to scan
-for references between multi-output derivations.
-
-This is sufficient to construct the referred store paths in each build output on
-the build client.