Add note about new Geo version upgrades page

What does this MR do?

From the discussion in Slack.

We have version specific upgrade instructions for Geo in two places:

• https://docs.gitlab.com/ee/update/ - General GitLab
• https://docs.gitlab.com/ee/administration/geo/replication/version_specific_upgrades.html - specific to Geo

Some page view stats (6th Dec 2022 - 4th Jan 2023):

• 168 - https://docs.gitlab.com/ee/administration/geo/replication/version_specific_upgrades.html
• 20k+ - https://docs.gitlab.com/ee/update/index.html

We do have a non-trivial volume of traffic to the Geo specific pages, so it's a good idea to have a message in the Geo specific page directing customers to also look at the main page until we consolidate these pages as tracked in &9581.

Related issues

&9581

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