Update documentation generation

[michaelbausor]

Explore what this looks like here: https://michaelbausor.github.io/gcloud-php/#/docs/google-cloud/master/core/upload/streamableuploader

Points to note:

• Classes now list superclasses and interfaces
• Classes include methods from superclasses and traits inside Google\Cloud
• Methods defined on external superclasses and traits are NOT included. This is because we cannot be sure they will have a docblock. We may want to consider including them anyway?
• Added a link the implementing class/trait on relevant methods
• Included links to build-in types and PSR classes/interfaces

Existing problem that isn't fixed: we don't link correctly when the docs specify a relative class name. See e.g. the param documentation in the existing docs (Duration and Timestamp): https://googlecloudplatform.github.io/google-cloud-php/#/docs/google-cloud/v0.34.1/spanner/database?method=execute
I am not sure what we should do about this - maybe nothing in this PR, but think about a future fix? The easiest thing is to change our docs so that we use FQNs everywhere, but that is ugly for someone reading the code.

[jdpedrie]

Existing problem that isn't fixed: we don't link correctly when the docs specify a relative class name.

Generally importing the class will fix this. I tested, and apparently it does not work correctly for nested types (i.e. members of the $options array). Agree that it can be addressed separately.

[dwsupplee]

Looks great!