Topic/syntax spec docs

[b-studios]

This PR ports the syntax spec from this tex document to Markdown and fixes a couple of typos and minor bugs.

@smarter What do you think?

[b-studios]

I purposefully reengineered the history to make it possible to review the changes. Of course I will squash the commits later to eventually merge.

[b-studios]

The two changes that might require reviewing are bac774d and cdc4442. All others are more or less boring.

[smarter]

Awesome! You should also add a link to it from https://github.com/lampepfl/dotty/blob/master/docs/docs/index.md, also the tex version should be deleted to avoid duplication.

[smarter]

The two changes that might require reviewing are bac774d and cdc4442.

@odersky, could you have a look please?

[b-studios]

Ah, one thing I was not sure about: I deleted the tree annotations that can be found in the tex version, like

-|  Refinement                                             RefinedTypeTree(EmptyTree, refinement)
+|  Refinement

Not sure, whether you want to keep them.

[odersky]

Looks great. Definitely better to have this in markdown.

I would leave the tree annotations in this version of the document, though. The reason is that they give us a good overview what parsed constructs map into. When we export the document to a language spec, they would be removed, of course.

[odersky]

LGTM, Thanks!