Update Agent Doc With Two Termination Fields

Issue - Docs: Add section for default_max_hours_before_... (#427520 - closed)

What does this MR do and why?

Add section in the docs for agent configuration attributes - default_max_hours_before_termination and max_hours_before_termination_limit in https://docs.gitlab.com/ee/user/workspace/gitlab_agent_configuration.html. As with the current epic, we are introducing new fields to agent_config file, and this needs to be documented.

MR acceptance checklist

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• We should update the table with two new fields, and also in the subsection add appropriate examples.
• We must also update relevant docs to highlight the expected behaviour of updating the agent configuration max_hours_before_termination_limit. Namely, it does not affect the max_hours_before_termination_limit of existing workspaces, meaning lowering the value cannot be used to terminate existing workspaces.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.