codegen: protoc-gen-tarantool-doc — Markdown reference plugin

Sibling Go plugin under cmd/protoc-gen-tarantool-doc that emits one
Markdown file per input .proto. Sections (omitted when empty):

  - Header (path, package, imports)
  - Messages (per-message description + field table:
    # | Field | Type | Label | Description)
  - Enums (value table)
  - Services (method table with unary/client/server/bidi label)

Field type cells render scalar names, full type names for message/enum
references, and `map<K, V>` for maps. Synthetic map-entry messages are
skipped. Leading comments preserved via SourceCodeInfo (squashed to a
single line inside table cells).

Built via `make build-doc`; sample output committed at
examples/docs/hello.md via `make gen-docs`. Smoke tests in
test/doc_test.lua build the plugin if needed and assert the expected
sections and labels appear. 403/403 luatest green.

Initial commit: protoc-gen-tarantool plugin + pb runtime

A protoc plugin (Go) and a pure-Lua + LuaJIT-FFI runtime that give
Tarantool a complete proto3 + gRPC stack. Two codegen modes (full
inline / runtime descriptor), 226-test luatest suite, 18-fixture
mainline-protoc interop corpus, JSON codec, well-known types,
gRPC client/server factories, runtime .proto parser, microbench
harness with allocation regression gate.

Covers PLAN.md M1-M5. Module is `pb` (not `protobuf`) to avoid
colliding with Tarantool's built-in encode-only `protobuf` module.