Document decsisions: global template partials, sub-charts deployed from global chart

Document the decisions made in #352 (closed):

• Use templates/_helper.tpl whenever possible, excluding forked charts.
• Use charts/gitlab/templates/_helper.tpl for all sub-charts under gitlab.
• Sub-charts will be deployed via the global chart, by individual enable/disable rather than deploying them directly. Reduced maintenance, increased usability.

Closes #352 (closed)

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