[BUGFIX] Remove prefix whitespace for markdown

The problem is the whitespace infront of markdown like ####. It has no effect.
This line removes whitespaces at the beginning.

````
 #### Extract values from an array by key

 The extbase version of indexed_search returns an array of the
 previous search, which cannot easily be shown in the input field
 of the result page. This can be solved.
````
to
````
#### Extract values from an array by key

The extbase version of indexed_search returns an array of the
previous search, which cannot easily be shown in the input field
of the result page. This can be solved.
````

Fix https://github.com/FluidTYPO3/vhs/issues/592
For proper CGL: `$documentation = preg_replace('/(^\ |$)/m', '', $documentation);`