Adds support for sub navigation main menu

[h-enk]

Supports one level deep.

Set in ./config/_default/menus/menus.en.toml:

[[main]]
  name = "Prologue"
  weight = 30
  identifier = "prologue"
  url = "/docs/prologue/"

[[main]]
  name = "Introduction"
  weight = 35
  identifier = "introduction"
  url = "/docs/prologue/introduction/"
  parent = "prologue"

[[main]]
  name = "Quick Start"
  weight = 40
  identifier = "quick-start"
  url = "/docs/prologue/quick-start/"
  parent = "prologue"

[[main]]
  name = "Commands"
  weight = 45
  identifier = "commands"
  url = "/docs/prologue/commands/"
  parent = "prologue"

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