Documentation examples out of date

[kimskimchi]

1. Links broken
   On the main page, the link to "Client Library Documentation" is broken.

2. Package name update
   In client documentatin, update examples to the correct import.

For example:
Old: from google.cloud import billing_budgets_v1beta1
New: from google.cloud.billing import budgets_v1beta1

[kimskimchi]

Seems that 2 will need a v1.0.0 client documentation release, as the current documentation is for 0.4.0