test: disable docs build in CI, for now (#32741)

@feanil is overhauling the docs structure of edx-platform currently.
Recently, a change he made upped the docs build time from <15mins
to >40mins. Rather than revert the changes he is making, we're going
to take the docs build check out of CI for now. He plans to re-enable
the build check, but as a ReadTheDocs build, not a GitHub Action.
He's actively working on the docs, so he's not worried about any
regressions that may pop up while the check is disabled.

It's worth noting that the current docs check does not actually
publish its output anywhere.

.github/workflows/docs-build-check.yml

@@ -1,50 +0,0 @@
-name: Docs build
-
-on:
-  pull_request:
-  push:
-    branches:
-      - master
-
-jobs:
-  tests:
-    name: Docs build
-    runs-on: ${{ matrix.os }}
-    strategy:
-      matrix:
-        python-version: ['3.8']
-        os: ['ubuntu-20.04']
-
-    steps:
-      - uses: actions/checkout@v2
-      - name: Set up Python
-        uses: actions/setup-python@v4
-        with:
-          python-version: ${{ matrix.python-version }}
-
-      - name: Install system requirements
-        run: sudo apt update && sudo apt install -y libxmlsec1-dev
-
-      - name: Install pip
-        run: python -m pip install -r requirements/pip.txt
-
-      - name: Get pip cache dir
-        id: pip-cache-dir
-        run: echo "::set-output name=dir::$(pip cache dir)"
-
-      - name: Cache pip dependencies
-        id: cache-dependencies
-        uses: actions/cache@v3
-        with:
-          path: ${{ steps.pip-cache-dir.outputs.dir }}
-          key: ${{ runner.os }}-pip-${{ hashFiles('requirements/edx/development.txt') }}
-          restore-keys: ${{ runner.os }}-pip-
-
-      - name: Install python dependencies
-        run: make dev-requirements
-
-      - name: Install docs requirements
-        run: pip install -r requirements/edx/doc.txt
-
-      - name: Docs build
-        run: make docs