docs: generate uniform service overviews

[stephenplusplus]

RE: #1471

@callmehiphop I wanted to get your thoughts before I move forward. The changes in this PR:

• Removes custom constructor descriptions. They mostly were filler, slight variants of something like "This is how you interact with a {serviceName} object".
• Removes constructor @example blocks. This was mostly used to show how to create a client-- that is now covered in the generated description.
• Creates a template file that should be used by all constructors for their description. This file should be parsed with _.template(), using a common templating syntax. This is done because each description should be different, depending on the context the user is viewing the docs in: umbrellaMode or not.
• Exports a function from scripts/docs.buildOverview.js that takes a class name, like bigquery, and returns the templated description. This returned description should be pre-pended to any existing description a constructor might have.

Constructor @example blocks are currently used to initialize variables that are used throughout the rest of the class's examples. Removing these blocks created a bit of a problem, so as a solution, in test/docs.js, it will prepend example code with a manual instantiation.

[callmehiphop]

Where would the overview content be visible to the user? In the collapsable menu like today? Or before the constructor description?

[stephenplusplus]

There is only one body of text that describes a service and its constructor. So the Getting Started and Overview sections are gone. All content would appear as part of the constructor's description.

[stephenplusplus]

This has been absorbed by #1479.

[stephenplusplus]

@callmehiphop

[stephenplusplus]

It looks like the overview changes from this PR weren't carried over into #1479. The docs site still show 3 different sections for each API;

• Getting Started
• Overview
• Constructor description

There is only meant to be one now, under the label of the constructor, i.e.:

[stephenplusplus]

Updated this PR, PTAL.

I also stuck Datastore constants on the prototype... it was never very convenient accessing them:

var datastore = require('@google-cloud/datastore')({ ... });
datastore.int // undefined (until now)

Are there other constants I need to do this for?

Until we get this change in, I don't think we should officially "release" (publishing the release notes), since the docs right now are pretty confusing, e.g. the first example on the BigQuery module's page tells you to require('google-cloud').

[coveralls]

Coverage remained the same at 100.0% when pulling 0e31d81 on stephenplusplus:spp--docs-build-prep into 66ac068 on GoogleCloudPlatform:master.

[callmehiphop]

@stephenplusplus LGTM, I think we should wait for an update to the docs site before merging this though so that our overview isn't collapsed.