docs(guide/expression): fix duplicate key error in example

The example that demonstrates how to parse expressions can fail if you
pass in the same expression twice.  By using "track by $index" we can
fix this.

Closes #4472
docs(guide/e2e-testing): remove superfluous block of text

The removed block is repeated from the unit-testing guide and appears
in an awkward place in this text.

Closes #4456
docs(guide): rewrite guide landing page

Change landing page to include a set of curated links with subject
headings as a guide through Angular's docs and other resources.

Closes #4470
docs(tutorial/step-5): fix wrong link to services guide

Closes #4477
style(injectorSpec): fix typo in "it" description

Closes #4483
docs(tutorial/step-2): add beforeEach to load module

The non-global controller test throws an error because the test does not
know about the module and so can not find the controller. This change
tells the test about the module so the test can find the controller.

Closes #4489