Adding separators after docstrings

[abogdanov85]

I have this problem when using DocumenterMarkdown.

If we have a block of several docstrings like this:

```@docs
Base.Function1
Base.Function2
```
Some text outside of macro.

then in the output we will get something like this:

<a id='Base.Function1' href='#Base.Function1'>#</a>
**`Base.Function1`** &mdash; *Function*.

Docstring text for Function1

<a id='Base.Function2' href='#Base.Function2'>#</a>
**`Base.Function2`** &mdash; *Function*.

Docstring text for Function2

Some text outside of macro.

The problem is that we don't have any separator between the end of docstring text and the next text. Because of that it is difficult to further isolate the docstring texts on this page (if we want to convert it to some other format and maybe to make some highlighting for every separate docstring - for example add frames to them).

It would be great to have something like this in the output:

<a id='Base.Function1' href='#Base.Function1'>#</a>
**`Base.Function1`** &mdash; *Function*.

Docstring text for Function1

[//]: # (End of docstring)

<a id='Base.Function2' href='#Base.Function2'>#</a>
**`Base.Function2`** &mdash; *Function*.

Docstring text for Function2

[//]: # (End of docstring)

Some text outside of macro.

Or any other type of separators (horizontal line, some html tag etc.)

[asinghvi17]

Will be solved by #13