Make `ExDoc.DocAST` public and document documentation AST

[eksperimental]

Since Road to v1.0 has been published, and not trying to pollute the thread there I'm opening this issue.

I think we need to make ExDoc.DocAST public and document the documentation AST format. I was looking into implementing a Markdown processor implementation and I have to read the source code to understand the format.

Also note that currentlyExDoc.DocAST.to_string/1 generates HTML code, so it is couple to HTML and not a total abstraction.

[josevalim]

The AST is pretty much a HTML AST, so it makes sense to_string returns HTML but we should probably rename it to to_html. Feel free to send a PR after #2148 is merged!

I merged this into the parent issue, thanks!