Tune up backend GHA (#495)

* run cargo steps in sequence

* reorder jobs and gha push event only for master

* split backend gha

.github/workflows/backend_docs.yml

@@ -0,0 +1,45 @@
+name: Backend CI - cargo doc
+
+on:
+  push:
+    branches:
+      - master
+    paths:
+      - '.github/workflows/backend*'
+      - 'backend/**'
+      - '!frontend/**'
+  pull_request:
+    paths:
+      - '.github/workflows/backend*'
+      - 'backend/**'
+      - '!frontend/**'
+
+env:
+  CARGO_TERM_COLOR: always
+
+defaults:
+  run:
+    working-directory: ./backend
+
+jobs:
+  docs:
+    name: Check Documentation
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout sources
+        uses: actions/checkout@v3
+
+      - name: Install Rust stable toolchain
+        uses: actions-rs/toolchain@v1
+        with:
+          profile: minimal
+          toolchain: stable
+          override: true
+
+      - name: Rust Cache
+        uses: Swatinem/rust-cache@6720f05bc48b77f96918929a9019fb2203ff71f8 # v2.0.0
+        with:
+          workspaces: backend
+
+      - name: Check internal documentation links
+        run: RUSTDOCFLAGS="--deny broken_intra_doc_links" cargo doc --verbose --workspace --no-deps --document-private-items