Build a custom reference index page

[arthur-shaw]

Closes #16

This PR:

• Uses a pkgdown-style YAML file to specify how command reference entries should be organized. See here.
• Constructs an index page

YAML format

Example using labeller

reference:
- title: "Variable labels"
  desc: "Manipulating value labels"
- subtitle: "Pipes"
  desc: "Sometimes SuSo uses pipes like `%rostertitle%`"
  contents:
  - lbl_list_pipes
  - lbl_replace_pipe
  - lbl_assert_no_pipes
- subtitle: "Length"
  contents:
  - lbl_list_long_varlbl
  - lbl_assert_no_long_varlbl
- subtitle: "Content"
  contents:
  - lbl_list_matching_vars
  - lbl_list_no_varlbl
  - lbl_assert_varlbls
- title: "Value labels"
  desc: "Manipulating value labels"
- subtitle: "Content"
  contents:
  - lbl_list_matching_vals

What's missing

Currently missing:

• [x] Build custom index page with end-to-end workflow
• [x] Manage whether to use default or custom page based on presence (absence) of reference_index.yml
• [x] Integrate code into main files
• [ ] Add tests using fixtures used for interactive testing