docs: Add a document about auto-upgrading dependencies of Cells

Related to gitlab-com/gl-infra/delivery#21223 (closed).

This is a document that attempts to summarize some of the implementation details of our current automatic dependency upgrade process for Cells. We already used to use it for Instrumentor, and now, we also use it for using the latest pre-release Helm charts. I have attempted to reduce the duplication between this document and the blueprint for this feature: https://gitlab.com/gitlab-com/gl-infra/gitlab-dedicated/team/-/blob/main/architecture/blueprints/cell-chart-enhancement.md.

I had a question which was answered in the MR comments by @skarbek:

question: Do we update the blueprint after implementation to reflect the final implementation? Or do we use use the blueprint primarily to discuss the implementation approach?

Normally we do not. Blueprints usually reflect decisions made at that time and they kinda showcase how we've iterated over time.

We should certainly reference the blueprint if it contains enough valuable information, but for overall documentation or runbook style of purposes, we'd want to keep them segregate.