Allows navbar templates to be added on a per-page basis

[choldgraf]

Once https://github.com/pydata/pydata-sphinx-theme/pull/355/files is merged, users can define the templates that go into the navbar via patterns like:

html_theme_options = {
  "navbar_start": ["list", "of", "templates"]
}

it'd be useful if this behaved similarly to html_sidebars, so something like:

html_theme_options = {
  "navbar_start": {
    "**": ["list", "of", "templates"],
    "apage": ["some", "other", templates"],
  }
}