Outline for BigPipe module guide

For context see #2887480: Outline, standardize, and complete documentation for Drupal 8 core modules.

Proposed outline

Here's a proposed outline for what the documentation for the guide at https://www.drupal.org/docs/8/core/modules/big_pipe should contain.

• Page: BigPipe module overview
  Module description, and example use cases. What does this module do? When should I enable it?

  Start by copy pasting from help text displayed by the help module at admin/help/big_pipe, and adjust.

• Related Page: BigPipe environment requirements

Next steps

• Review above outline and suggest changes
• Review and update existing overview page
• Draft pages in guide and publish for review