Additions to prompts for docs MR reviews

Duo's technical writing reviews could improve if it could ingest the contents of the style guide. Apparently, they can't be accessed over a HTTP(S) connection as part of the Duo review process, but the files are also available locally in this project. What if, instead, we pointed to the local location of the files, in terms of the current repo? Would that make the information available to the prompt?

This MR adds two sets of instructions:

1. Adds 5. Adhere to the documentation style guide, available in 'doc/development/documentation/styleguide/' to the general prompt for files in the doc/* directory.
2. Adds a new section matching files in the API docs directory, and instructs Duo to append the RESTFUL style guide:

  - name: API documentation
    fileFilters:
      - "doc/api/**/*.md"
    instructions: |
      1. User-facing text in API pages should adhere to the RESTFUL documentation style guide, available in 'doc/development/documentation/restful_api_styleguide.md'

Will it improve matters? I don't know, but in theory it doesn't hurt to ask, and the API style recommendations have been incorrect so far.

Thoughts?