Document nunjucks.parser

[ogonkov]

It's a hidden treasure, and it would be nice to have some docs for nunjucks.parser and nunjucks.nodes.*.

You could guess the meaning of some nunjucks.nodes, but having docs would be much better.

[fdintino]

I agree this is a good idea, but some of the API will likely change for v4 of nunjucks (currently in-progress). I'll write some docs for it as part of the release.

[ogonkov]

@fdintino where i can track v4 progress? develop branch?

[fdintino]

@ogonkov that is where I'll be pushing out changes. I have a bunch of work locally that I need to clean up and push out. I may find time to do it this weekend.

[ogonkov]

Should be done after #1059