- include `.ruby-version` and in the project root (and unignore in
`.gitignore`) to improve editor support which expects these files
- use the `.ruby-version` in `docs/Gemfile` to use a consistent Ruby
version here
- remove hardcoded Ruby versions from `workflows/docs` in favour of
using the `.ruby-version` file instead
- use more consistent configuration in `workflows/docs` to match
`workflows/tests`
