docs: Fix markdown lint MD012 and MD022

MD012 Multiple Consecutive Blanks
https://github.com/DavidAnson/markdownlint/blob/master/doc/Rules.md#md012---multiple-consecutive-blank-lines

MD022 Headings should be surrounded by blank lines
https://github.com/DavidAnson/markdownlint/blob/master/doc/Rules.md#md022---headings-should-be-surrounded-by-blank-lines

docs/sequenceDiagram.md

@@ -77,7 +77,6 @@ Type | Description
 -x   | Solid line with a cross at the end (async)
 --x  | Dotted line with a cross at the end (async)
 
-
 ## Activations
 
 It is possible to activate and deactivate an actor. (de)activation can be dedicated declarations:
@@ -127,7 +126,6 @@ sequenceDiagram
     John-->>-Alice: I feel great!
 ```
 
-
 ## Notes
 
 It is possible to add notes to a sequence diagram. This is done by the notation
@@ -159,7 +157,6 @@ sequenceDiagram
     Note over Alice,John: A typical interaction
 ```
 
-
 ## Loops
 
 It is possible to express loops in a sequence diagram. This is done by the notation
@@ -187,7 +184,6 @@ sequenceDiagram
     end
 ```
 
-
 ## Alt
 
 It is possible to express alternative paths in a sequence diagram. This is done by the notation
@@ -280,8 +276,8 @@ sequenceDiagram
     end
 ```
 
-
 ## Background Highlighting
+
 It is possible to highlight flows by providing colored background rects. This is done by the notation
 
 The colors are defined using rgb and rgba syntax.
@@ -386,10 +382,8 @@ loopLine     | Defines styles for the lines in the loop box.
 note         | Styles for the note box.
 noteText     | Styles for the text on in the note boxes.
 
-
 ### Sample stylesheet
 
-
 ```css
 body {
     background: white;
@@ -472,7 +466,6 @@ text.actor {
 }
 ```
 
-
 ## Configuration
 
 Is it possible to adjust the margins for rendering the sequence diagram.