Adding get doc content endpoint for IDE custom templates

[TalkySafe143]

I'm actually working on cpinitiative/ide#33

And I've been thinking, that while the template can be loaded to the new file with the copyFile endpoint, I think it would be much lighter if the content of the file was retrieved from the template.

With the content it is much easier to reset the file to the template which is a functionality that is currently with the default code of each file.

I propose to add an endpoint where given a file it returns the content in plain text. So, when the page is rendering, retrieve the template and only set the value for the editor, the handleInsertFileTemplate can be reused.