Respect sidebar_position in sidebar ordering

Closes #163

The custom logic we use to order the Docusuarus-generated sidebar does
not respect the `sidebar_position` frontmatter field. To fix this,
retrieve the `sidebar_position` value from docs page sidebar entries.
For a sidebar category, retrieve the category index page and use its
`sidebar_position`.

The ordering logic now works like this:
- Insert items with a predefined sidebar position at the required
  indices within the final sidebar item array.
- Insert the remaining sidebar items into the empty slots in the final
  array. Order these items in the usual order, sorting alphabetically by
  title (or category label) and placing items with "Introduction" and
  "Get(ting)? Started" before alphabetized items.

Author: ptgott