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).