add_resource DSL now available for builders

[jaredcwhite]

Summary

This provides a DSL for builder plugins to add resources, much like the past era doc DSL. For example:

def build
  add_resource :posts, "im-a-markdown-post.md" do
    title "I'm a Markdown post!"
    resolve_me from: -> { method_value }
    delayed -> { 123 }
    nested do
      val 456
    end
    content <<~MARKDOWN
      # Hello World!
    MARKDOWN
  end
end

There are a few added niceties compared to the previous doc as well as a few slight changes, but generally it should work and feel pretty much the same.

One cool thing now that we're in the resources era is if you add def self.resource_data_for_id(id) to your builder, you can recreate the resource via Bridgetown::Model::Find!

def build
  # creates id: builder://MyBuilder/later.html
  add_resource :page, "later.html" do
    title "Later, alligator!"
  end
end

def self.resource_data_for_id(id)
  { title: "After a while, crocodile!" } if id == "builder://MyBuilder/later.html"
end

later:

resource = Bridgetown::Model::Base.find("builder://MyBuilder/later.html").as_resource_in_collection

resource.data.title == "After a while, crocodile!" # true

(That would prove useful in any scenario in which you're adding a whole bunch of resources based on external API data.)

Tests have been added and passing, documentation will be forthcoming as part of the overall website revamp.

Context

Closes #332

[render]

Your Render PR Server URL is https://bridgetown-beta-pr-419.onrender.com.

Follow its progress at https://dashboard.render.com/static/srv-c5k7pk7d17cbcdh94agg.

[render]

Your Render PR Server URL is https://bridgetown-api-pr-419.onrender.com.

Follow its progress at https://dashboard.render.com/static/srv-c5k7pknd17cbcdh94an0.