Add responsibilities and workflow for stage leads

Why is this change being made?

Adding some details for Technical Writing stage leads and how we anticipate they will spend their time and plan feature work.

• As much as possible, I think we should adapt existing processes rather than trying to create new ones. I've suggested some tweaks for the feature development workflow.
• We need to define a stage lead, but without it sounding like a whole new role. The definition should probably live here?: https://handbook.gitlab.com/job-families/product/technical-writer/ Add the definition to the workflow page. This is not a new role, it's a different focus/flow of work.
• Still to do: figure out a workflow for new/enhanced content that stage leads will drive. This is not the same as backlog work.

Author and Reviewer Checklist

Please verify the check list and ensure to tick them off before the MR is merged.

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