Render documentation for provider init callbacks

[tetromino]

If a provider has a custom init callback, we want the summary blurb to show init's parameters (since these are what the user will interact with); we have to render constructor paramaters and fields separately (and using separate html anchors) since there is not necessarily a 1-1 relationship between them, and since they may have different docs.

Fixes #182

[tetromino]

Current PR is not ready. I need to fix the following problems which occurred to me in the process of testing it:

• In virtually all cases, the init callback's summary docstring is not interesting to the user (it's along the lines of "Validates params foo and bar") - so we should not render it. The provider's docstring is the interesting part for the user.
  • done
• Typically, the list of parameters of the init callback exactly matches the list of fields. If that is so, we should merge parameters and fields into a single table.
• And even if the set of init parameters is different from fields, we should propagate docstrings from fields to undocumented init parameters with the same name.

In other words, fairly extensive munging of the ProviderInfo proto is necessary.

[tetromino]

I think by default, we want the following behavior:

• Custom init callback specified
  • The set of parameters for the init callback equals the set of
    fields for the provider; and the docs for the init callback's
    parameters are either empty or equal to corresponding field docs
    • Some init parameters have a default value:
      -> Render a single "Fields" table with 3 columns (name, doc,
      default value)
    • ... otherwise
      -> Render a single "Fields" table with 2 columns
  • ... otherwise
    -> Render two tables - "Constructor parameters" and "Fields" - with
    the links from the summary blurb (interfixed with "_init")
    leading to the parameters table (not the fields table)
• ... otherwise
  -> Trivial case - single "Fields" table (as before).

[brandjon]

Discussed offline, approved but mind the rendering failure in one of the goldens,