Rename the 'sub-tutorials' to 'guides'

[brson]

This was a good suggestion @steveklabnik made a while ago. 'Guides' is a better description for these sorts of deep dives. This will involve changing the filenames, changing the text of the guides, probably changing some of the text at the end of the tutorial that links to them, and also changing how the doc wiki page is organized (for the master branch only).

[steveklabnik]

Yup!

We should consider re-merging in my old PR for this.

[alan-andrade]

Hey @steveklabnik , what's the PR your talking about? I'm going to dig into this.

[steveklabnik]

@alanandrade #9579

[alan-andrade]

This one too. I think it should be closed.

[pnkfelix]

part of #11755

(hopefully I'll get a chance after this pass building the metabug to check if this issue itself should be closed as alan-andrade suggests.)

[huonw]

Closed by #11106.