Standardize capitalization of Runner in docs

Problem to solve

In both the UI and the documentation, we are inconsistent in the capitalization of the term "runner" as well as "shared runner," "group runner," and "specific runner."

Further details

We recently updated the style guide to lean toward lowercase. https://docs.gitlab.com/ee/development/documentation/styleguide.html#feature-names

Proposal

Here is proposed usage for these terms.

Term	Usage
GitLab Runner	Capitalize only when talking about the installed software. For example: Install GitLab Runner on Linux. Do not use the plural "GitLab Runners."
runner	Use lowercase any other time you're talking about runners. For example: Start the runner. Update the runner. Use the runner to execute the job in your pipeline.
runners	Use lowercase. For example: View the list of runners.
shared runner	Use lowercase. Plural is fine too. For example: Enable a shared runner. View shared runners.
group runner	Use lowercase. Plural is fine too. For example: Enable a group runner. View group runners.
specific runner	User lowercase. Plural is fine too. For example: Enable a specific runner. View specific runners.

Who can address the issue

Other links/references