Update tutorial links after big split

[blink1073]

The link check is failing in master, causing current PRs to fail:

+py.test --check-links -k .md .
============================= test session starts ==============================
platform linux -- Python 3.6.0, pytest-3.0.7, py-1.4.33, pluggy-0.4.0
rootdir: /home/travis/build/jupyterlab/jupyterlab, inifile: pytest.ini
plugins: check-links-0.1.0
collected 113 items 
CONTRIBUTING.md ..........
README.md .....................
RELEASE.md ..
design/about.md .
tutorial/adding_content.md .FF.
tutorial/documents.md ..........
tutorial/examples.md ..
tutorial/labextensions.md ..
tutorial/notebook.md ..................
tutorial/patterns.md ..
tutorial/plugins.md F.FFFFFFFFFF.
tutorial/repo.md .
tutorial/virtualdom.md ..
=================================== FAILURES ===================================
 /home/travis/build/jupyterlab/jupyterlab/tutorial/adding_content.md: https://github.com/jupyterlab/jupyterlab/blob/master/typings/ansi_up/ansi_up.d.ts 
https://github.com/jupyterlab/jupyterlab/blob/master/typings/ansi_up/ansi_up.d.ts: HTTP Error 404: Not Found
 /home/travis/build/jupyterlab/jupyterlab/tutorial/adding_content.md: https://github.com/jupyterlab/jupyterlab/blob/master/typings/xterm/xterm.d.ts 
https://github.com/jupyterlab/jupyterlab/blob/master/typings/xterm/xterm.d.ts: HTTP Error 404: Not Found
 /home/travis/build/jupyterlab/jupyterlab/tutorial/plugins.md: https://github.com/jupyterlab/jupyterlab/tree/master/examples/lab 
https://github.com/jupyterlab/jupyterlab/tree/master/examples/lab: HTTP Error 404: Not Found
 /home/travis/build/jupyterlab/jupyterlab/tutorial/plugins.md: https://github.com/jupyterlab/jupyterlab/blob/master/src/terminal-extension/index.ts 
https://github.com/jupyterlab/jupyterlab/blob/master/src/terminal-extension/index.ts: HTTP Error 404: Not Found
 /home/travis/build/jupyterlab/jupyterlab/tutorial/plugins.md: https://github.com/jupyterlab/jupyterlab/blob/master/src/shortcuts-extension/index.ts 
https://github.com/jupyterlab/jupyterlab/blob/master/src/shortcuts-extension/index.ts: HTTP Error 404: Not Found
 /home/travis/build/jupyterlab/jupyterlab/tutorial/plugins.md: https://github.com/jupyterlab/jupyterlab/blob/master/src/imagewidget-extension/index.ts 
https://github.com/jupyterlab/jupyterlab/blob/master/src/imagewidget-extension/index.ts: HTTP Error 404: Not Found
 /home/travis/build/jupyterlab/jupyterlab/tutorial/plugins.md: https://github.com/jupyterlab/jupyterlab/blob/master/src/help-extension/index.ts 
https://github.com/jupyterlab/jupyterlab/blob/master/src/help-extension/index.ts: HTTP Error 404: Not Found
 /home/travis/build/jupyterlab/jupyterlab/tutorial/plugins.md: https://github.com/jupyterlab/jupyterlab/blob/master/src/filebrowser-extension/index.ts 
https://github.com/jupyterlab/jupyterlab/blob/master/src/filebrowser-extension/index.ts: HTTP Error 404: Not Found
 /home/travis/build/jupyterlab/jupyterlab/tutorial/plugins.md: https://github.com/jupyterlab/jupyterlab/blob/master/src/editorwidget-extension/index.ts 
https://github.com/jupyterlab/jupyterlab/blob/master/src/editorwidget-extension/index.ts: HTTP Error 404: Not Found
 /home/travis/build/jupyterlab/jupyterlab/tutorial/plugins.md: https://github.com/jupyterlab/jupyterlab/blob/master/src/console-extension/index.ts 
https://github.com/jupyterlab/jupyterlab/blob/master/src/console-extension/index.ts: HTTP Error 404: Not Found
 /home/travis/build/jupyterlab/jupyterlab/tutorial/plugins.md: https://github.com/jupyterlab/jupyterlab/blob/master/src/services-extension/index.ts 
https://github.com/jupyterlab/jupyterlab/blob/master/src/services-extension/index.ts: HTTP Error 404: Not Found
 /home/travis/build/jupyterlab/jupyterlab/tutorial/plugins.md: https://github.com/jupyterlab/jupyterlab/blob/master/src/rendermime-extension/index.ts 
https://github.com/jupyterlab/jupyterlab/blob/master/src/rendermime-extension/index.ts: HTTP Error 404: Not Found
 /home/travis/build/jupyterlab/jupyterlab/tutorial/plugins.md: https://github.com/jupyterlab/jupyterlab/blob/master/src/docregistry-extension/index.ts 
https://github.com/jupyterlab/jupyterlab/blob/master/src/docregistry-extension/index.ts: HTTP Error 404: Not Found
============================= 25 tests deselected ==============================
============= 13 failed, 75 passed, 25 deselected in 8.35 seconds ==============

[blink1073]

Most likely the links will need to be removed since we lost our API docs build temporarily, pending support in TypeDoc.

[blink1073]

cf TypeStrong/typedoc#449