Define Runner terminology

There are several terms that are specific to GitLab Runner that we need to agree on before we can update the documentation.

1. Confirm definitions in this issue.
2. @fneill to update style guide word list to ensure words are used consistently in the docs.
3. Add terms to the docs, using the glossary topic type.

Here are a list of terms (definitions for GitLab Runner and Worker were pulled from this MR).

Please add terms and definitions for any other items that should be described in the docs.

Term	Definition/Usage
GitLab Runner	GitLab Runner is the application you install that executes GitLab CI jobs on a target computing platform.
runner	The agent that runs the code on the host platform and displays in UI. If a runner is registered with the same token, the runner could represent a collection of runners and runner managers.
runner manager	A type of runner that can create multiple runners for autoscaling. Specific to the type of executor used.
runner machine