minor #15257 Verify code blocks (Nyholm)

This PR was squashed before being merged into the 4.4 branch.

Discussion
----------

Verify code blocks

This PR is a first step to make sure we are using valid code blocks. I created a [small parser](https://github.com/symfony-docs-tools/code-block-checker) that will replace [the PR to the docs builder](https://github.com/weaverryan/docs-builder/pull/97).

This CI job will look at the modified files, and give the the the [code-block-checker](https://github.com/symfony-docs-tools/code-block-checker). That project will find the code blocks and make sure they are using valid syntax. It also supports a baseline so we can write invalid syntax if we want to. (it may sometimes make the examples more readable See #15250)

~~The next step is to make sure one can actually run code examples. Im planning to get all examples that starts with a comment and figure out if that is a filepath to the config directory. I will then capture any symfony errors when using that config.~~ This is super helpful when updating the PHP config to use the config builders.

EDIT: I included "the next step" in this PR. We do run all config examples.

Note that thanks to the baseline feature, will only report **new** errors on modified code blocks.

Commits
-------

c92641d Verify code blocks

Author: javiereguiluz

.github/workflows/ci.yaml

@@ -103,3 +103,63 @@ jobs:
                 uses: docker://oskarstark/doctor-rst
                 with:
                     args: --short --error-format=github --cache-file=/github/workspace/.cache/doctor-rst.cache
+
+    symfony-code-block-checker:
+        name: Code Blocks
+        runs-on: Ubuntu-20.04
+        continue-on-error: true
+        steps:
+          - name: Checkout code
+            uses: actions/checkout@v2
+
+          - name: Set-up PHP
+            uses: shivammathur/setup-php@v2
+            with:
+                php-version: 8.0
+                coverage: none
+
+          - name: Fetch branch from where the PR started
+            run: git fetch --no-tags --prune --depth=1 origin +refs/heads/*:refs/remotes/origin/*
+
+          - name: Find modified files
+            id: find-files
+            run: echo "::set-output name=files::$(git diff --name-only origin/${{ github.base_ref }} HEAD | grep ".rst" | tr '\n' ' ')"
+
+          - name: Get composer cache directory
+            id: composercache
+            run: echo "::set-output name=dir::$(composer config cache-files-dir)"
+
+          - name: Cache dependencies
+            uses: actions/cache@v2
+            with:
+                path: ${{ steps.composercache.outputs.dir }}
+                key: ${{ runner.os }}-composer-${{ hashFiles('**/composer.lock') }}
+                restore-keys: ${{ runner.os }}-composer-
+
+          - name: Install dependencies
+            if: ${{ steps.find-files.outputs.files }}
+            run: |
+              git clone --depth 15 https://github.com/symfony-docs-tools/code-block-checker.git ../_checker
+              cd ../_checker
+              composer install
+
+          - name: Install test application
+            if: ${{ steps.find-files.outputs.files }}
+            run: |
+              git clone -b ${{ github.base_ref }} --depth 5 --single-branch https://github.com/symfony-docs-tools/symfony-application.git ../_sf_app
+              cd ../_sf_app
+              composer update
+
+          - name: Generate baseline
+            if: ${{ steps.find-files.outputs.files }}
+            run: |
+              CURRENT=$(git rev-parse HEAD)
+              git checkout -m ${{ github.base_ref }}
+              ../_checker/code-block-checker.php verify:docs `pwd` ${{ steps.find-files.outputs.files }} --generate-baseline=baseline.json --symfony-application=`realpath ../_sf_app`
+              git checkout -m $CURRENT
+              cat baseline.json
+
+          - name: Verify examples
+            if: ${{ steps.find-files.outputs.files }}
+            run: |
+              ../_checker/code-block-checker.php verify:docs `pwd` ${{ steps.find-files.outputs.files }} --baseline=baseline.json --output-format=github --symfony-application=`realpath ../_sf_app`