EE: Refactor the upgrading from source docs

What does this MR do?

This is the EE version of https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/25003

Related issues

Closes https://gitlab.com/gitlab-org/release/framework/issues/190

Author's checklist

• Apply the correct labels and milestone
• Crosslink the document from the higher-level index
• Crosslink the document from other subject-related docs
• Port the MR to EE (or backport from CE): always recommended, required when the ee-compat-check job fails

Review checklist

• Your team's review (required): @marin
• Technical writer's review (required): @axil / @marcia
• Merge the EE-MR first, CE-MR afterwards