Docker executor image clarification and macOS virtualization info

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What does this MR do?

docs/executors/docker.md

1. It clarifies which file takes precedence when an image is specified in the config.toml and .gitlab-ci.yml file.
2. It adds macOS to a table showing which platform a Runner can be installed on and if it supports a Docker executor.
3. It adds that a gitlab-runner command can automatically populate new configs to the config.toml file.
4. It properly formats all tables.

docs/install/osx.md

1. It adds a note of admonition stating that macOS users who have a GitLab Runner installed can run GitLab on a container or VM locally or remotely.
2. It moves the troubleshooting link notice to the troubleshooting section and also makes sure the sentence aligns with the style guide.

Why was this MR needed?

• The issue in docs/executors/docker.md was raised by a contributor, and the user is correct.
• macOS users can run GitLab locally and use a Docker executor. See Lab-04 below.

What's the best way to test this MR?

See Lab-04 and Lab-05 of my project.

What are the relevant issue numbers?

gitlab#358403 (closed)