Adjust nginx headings to fix hierarchy (#552)

[Hooverdan96]

Fixes #552

This pull request's proposal

Adjust heading levels to fix cross-reference links in Rock-ons with writeups toctree

Checklist

• With the proposed changes no Sphinx errors or warnings are generated.
• I have added my name to the AUTHORS file, if required (descending alphabetical order).

[phillxnet]

@Hooverdan96 Thanks for seeing to this. Much appreciated.

Before

• nginx Rock-on
  -- What is nginx
  -- nginx Documentation
  -- Installing nginx Rock-on
• Advanced nginx configuration
  -- Introduction
• Using nginx commands
• Docker command examples for nginx Rockon

After

Using a production like instance to test rendering:

• nginx Rock-on
  -- What is nginx
  -- nginx Documentation
  -- Installing nginx Rock-on
  -- Advanced nginx configuration
  -- Docker command examples for nginx Rockon

That's better.

[phillxnet]

PRODUCTION published