DOC: edit Learn page content, and link to numpy.org/doc

[bjnath]

Adds a prominent link to the NumPy docs and clearly describes the remainder of the
page as external.

Adds a link to the contribution info at bottom and gives it a heading.

This page is still named learn.md! Someone will need to make the plumbing changes for this to become the Documentation page.

Addresses part of issue #235.

[rgommers]

I can't find the main discussion now, it's not gh-235.

I had another look at how other sites do this.

TensorFlow:

Pytorch:

Julia:

Jupyter:

I feel TensorFlow does it best: the split between Learn and API is clear, although it should probably be called API docs, and it gets you to where you want to go the fastest.

[rgommers]

I agree that most of the textual changes are an improvement. I'm not in favor of naming this page "Documentation", since it clearly isn't. The learning/docs split is common, and we can't really rework it properly before launch. So I think I want to revert that change and merge the rest.

[bjnath]

Learn vs Documentation
It's entirely about the users. If this is common practice and people can tell the difference at a glance it's not a problem.

I don't think the world has gotten there yet and we shouldn't be justifying our mistakes by looking at other sites' mistakes, but the call has been made, at least for now.

Everything on the page is documentation
As far as "clearly isn't" documentation, our own how-to guide calls it documentation:

The Documentation for a software project is the set of reference, instructional, educational, informative material generated by the project developers and contributors, as well as discussions, presentations, videos and other user-generated content.

Documentation typically comes from the project, but there's nothing in the word "documentation" that limits it to being project-generated.

[bjnath]

Issue reference should have been gh-234, not 235; sorry for the confusion.

[rgommers]

Rephrases are nice, thanks @bjnath. I'll push a couple of small tweaks and will then merge.

[rgommers]

In it goes, thanks @bjnath

[bjnath]

minor: is putting the comma within the quotes normal style? looks a little odd to me

I was inconsistent here; the period should be inside "Python+SciPy." too.

Per Google style guide:

[rgommers]

Okay, I'll make these last suggested changes both here and on the data science tab.

[bjnath]

If we're talking last changes before launch, I'd urge changing "Get started" to "Learn NumPy."

[rgommers]

Ah yes, will look at that issue too.