docs: Streamlined

[jaredcwhite]

Bridgetown 2.0 will ship with Serbea 2.0 which brings a new pipe feature to ERB files.

In addition, we will ship with Streamlined, a new gem which provides full HTML templating within pure Ruby using heretics and block-based string helpers.

There's also a new render_with do...end syntax for Roda file-based routes that uses the Ruby front matter DSL…very kewl.

These features merged in via #817, now just need to write the documentation.