Enable markdownlint rule (MD009) (#40887)

* Create markdownlint.yml

* Create markdownlint-problem-matcher.json

* Create .markdownlint.json

* Update .markdownlint.json

* fix violations

* fixes

* Remove "push" section

As advised by @viktorhofer so it's quite clear it only runs in CI.

Co-authored-by: Dan Moseley <danmose@microsoft.com>

.github/workflows/markdownlint-problem-matcher.json

@@ -0,0 +1,17 @@
+{
+    "problemMatcher": [
+        {
+            "owner": "markdownlint",
+            "pattern": [
+                {
+                    "regexp": "^([^:]*):(\\d+):?(\\d+)?\\s([\\w-\\/]*)\\s(.*)$",
+                    "file": 1,
+                    "line": 2,
+                    "column": 3,
+                    "code": 4,
+                    "message": 5
+                }
+            ]
+        }
+    ]
+}

.github/workflows/markdownlint.yml

@@ -0,0 +1,26 @@
+name: Markdownlint
+
+on:
+  pull_request:
+    paths:
+      - "**/*.md"
+      - ".markdownlint.json"
+      - ".github/workflows/markdownlint.yml"
+      - ".github/workflows/markdownlint-problem-matcher.json"
+
+jobs:
+  lint:
+
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: actions/checkout@v2
+    - name: Use Node.js
+      uses: actions/setup-node@v1
+      with:
+        node-version: 12.x
+    - name: Run Markdownlint
+      run: |
+        echo "::add-matcher::.github/workflows/markdownlint-problem-matcher.json"
+        npm i -g markdownlint-cli
+        markdownlint "**/*.md"

.markdownlint.json

@@ -0,0 +1,6 @@
+{
+    "default": false,
+    "MD009": {
+        "br_spaces": 0
+    },
+}

src/libraries/System.Text.Json/docs/KeyConverter_spec.md

@@ -129,7 +129,7 @@ The custom `KeyConverter` that calls Utf8Parser underneath performs slightly fas
 
 # Prior-art
 ## Newtonsoft.Json
- 
+
 ### On write:
 
 * if the `TKey` is a concrete primitive type*:
@@ -140,16 +140,16 @@ The custom `KeyConverter` that calls Utf8Parser underneath performs slightly fas
     * Double (uses `double.ToString("R")`) // 'R' stands for round-trip
     * Single
     * Enum (uses an internal helper method)
- 
+
 * If the `TKey` is `object` or non-primitive.
   * it calls the `TypeConverter` of the `TKey` runtime type.
   Except for :
     * `Type`, which returns the `AssemblyQualifiedName`.
   * If the type does not have a `TypeConverter`, it calls `ToString()` on the `TKey` instance.
 
- 
-\* A *primitive type* is a value cataloged as such by Json.Net from [this list](https://github.com/JamesNK/Newtonsoft.Json/blob/a31156e90a14038872f54eb60ff0e9676ca4a0d8/Src/Newtonsoft.Json/Utilities/ConvertUtils.cs#L119-L168). 
-  
+
+\* A *primitive type* is a value cataloged as such by Json.Net from [this list](https://github.com/JamesNK/Newtonsoft.Json/blob/a31156e90a14038872f54eb60ff0e9676ca4a0d8/Src/Newtonsoft.Json/Utilities/ConvertUtils.cs#L119-L168).
+
 ### On read:
 
 * If the `TKey` is a concrete type.