Add comment to help future users

What does this MR do?

Adds a comment to .gitlab/ci/_common.gitlab-ci.yml to remind users to update a corresponding docs page. Let's prevent a broken pipeline (docs:check development docs Go version job) in addition to testing for mismatches between the two pages.

Why was this MR needed?

In !5054 (comment 2152993175), I had a pipeline fail because GO_VERSION in .gitlab/ci/_common.gitlab-ci.yml did not match the version numbers in the commands in docs/development/index.md.

What's the best way to test this MR?

Code comments only; I don't think a test is needed?

What are the relevant issue numbers?

• Related to Break apart Helm chart optional config into new... (!5054 - merged) where this happened to me
• Related to technical-writing#1075 which was why I worked on Runner pages in the first place