Refactor code used to insert sections into ClojureDocs buffer

[katomuso]

I've moved the code used to insert sections into separate functions, as it is easier to modify sections that way. Maybe, in the future, I will also create a generic function to insert sections, so we can reduce boilerplate.

In places where we need to insert an empty line, I've used newline explicitly instead of the previously used additional \n in content insertions. This way, it will be more obvious where we are inserting actual content and where we are inserting just empty lines.

[bbatsov]

Thanks!