SIP MD to HTML translation issues

[soronpo]

I don't know if this is fixable, but currently there is unwanted result where a SIP MD file is translated to HTML and looks different. I noticed the following issues:

1. Acceptance of the ``` {code} ``` code annotation, and more specifically a ```scala {code} ``` annotation. Currently the only way to achieve the wanted result for the site is to use a {% highlight scala %} {code} {% endhighlight %} annotation. If we don't add ```, then it will look horrible on GitHub or any MD editor, but this adds extra viewable unwanted ``` to the site. The alternative is to use a 4-space prefix to the code. Unfortunately, this make all code on the site to have extra 4 space characters.
2. MD accepts ##Subject, but the the site translation expects space ## Subject.
3. MD accepts

---
# Subject

The site expects an empty line

---

# Subject

[SethTisue]

is this still applicable on the current site?

[Philippus]

Let's check with #1487 and update the https://github.com/scala/docs.scala-lang/blob/master/_sips/sip-template.md accordingly.

[NthPortal]

The by-name implicits SIP uses regular GFM markdown blocks, and renders fine on the site (link).