Backporting changes to the documentation committed directly to kamal-site

[kpumuk]

Several documentation updates to Kamal configuration were made directly in the kamal-site repository:

• Commits directly to kamal-2 branch, like basecamp/kamal-site@5fa8538
• Adds multiple hosts documentation kamal-site#102
• Fixing secret helpers link kamal-site#104
• Proofreading and minor corrections kamal-site#115
  (see full history here)

This MR backports the changes, and also:

• Updates bin/doc to produce better spacing for code fences
• Introduces a header into generated files to (hopefully) prevent editing them directly
• Updates the filename for the configuration overview page

The front matter will look like this:

# This file has been generated from the Kamal source, do not edit directly.
# Find the source of this file at lib/kamal/configuration/docs/accessory.yml in the Kamal repository.
title: Accessories

Closes #1003