Support nested sections

[kasium]

Let's imagine the following project X:

• frontend
  • fixed
  • features
• backend
  • fixed
  • features

Currently in towncrier you would need 4 fragments with their configuration in TOML. However, it would be a great feature if towncrier would support something like

[tool.towncrier]
package = "X"

  [[tool.towncrier.category]]
  directory = "backend"
  name = "Backend"

    [[tool.towncrier.type]]
    directory = "feature"
    name = "Features"
    showcontent = true

With that, given a file foo.backend.feature would render similar to

Backend
--------

Features
^^^^^^^^

* My feature (foo)

[chadrik]

This is supported in change as a feature called "components", and it's quite nice.

[SmileyChris]

This is already a feature, it's just not documented.
In #483, I've added some docs.

[kasium]

Great , thanks a lot!