Clarify how to configure network mode with docker executor

What does this MR do?

When working this customer ticket, I found it challenging to find information on where network_mode should be defined by using Algolia search.

Algolia only found one mention of network_mode and lead me to https://docs.gitlab.com/runner/executors/docker.html#networking page. The informaion on where exactly network_mode should be defined is present on The runner docker section page.

This MR tries to improve discoverability of The runner docker section from Networking page.

Related issues

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