Add documentation about how web terminals are secured

We have been used a few times for example here & here on how we secure the web terminal access and I think it would be good to add some documentation for the users to understand how everything is connected and even for future contributes/developers to GitLab to understand. The documentation should extend the already existing page for web terminals

• Documents Feature A
• Follow-up from: gitlab-org/gitlab-ce#50144, gitlab-runner!934 (merged)

New doc or update?

• New documentation
• Update existing documentation

Checklists

Product Manager

• Add the correct labels
• Add the correct milestone
• Indicate the correct document/directory for this feature
• Fill the doc blurb below

Documentation blurb

• Doc title

• Feature overview/description

• Feature use cases

Developer

• Copy the doc blurb above and paste it into the doc
• Write the tutorial - explain how to use the feature
• Submit the MR using the appropriate MR description template