Surface improvements to tutorials on the website more quickly

[zeke]

Problem

The website currently displays docs from the latest stable version of Electron. The stable version doesn't change very often, and sometimes there are weeks of beta versions before the next stable bump.

While it makes sense for the website's API documentation to strictly match the current stable version of Electron, the tutorial content found in docs/tutorial is generally not Electron-version-specific.

Currently we fetch all docs from latest:

Files	Source Commitish
/docs/**/*.md	electron@latest
locale.yml	electron.atom.io#master

Instead, I propose that we fetch docs/tutorial/** straight from Electron's master branch:

Files	Source Commitish
/docs/api/**/*.md	electron@latest
/docs/development/**/*.md	electron@latest
/docs/tutorial/**/*.md	electron#master
locale.yml	electron.atom.io#master

cc @leo, whose eagerness to see electron/electron#10251 deployed on the website prompted this idea.

[leo]

Very good idea! 👍😊