Update contributing guide with a few additional links

What does this MR do?

Updates links for a few topics that appeared to be intended to be links in the "contributing" guide.

I don't see any reference to a link syntax such as [topic name][some-path].

(I'm just reading the contributor docs to learn how to start working on Gitlab, and I noticed a few things that looked like they were supposed to be linked. Let me know if these are being transformed elsewhere and I shouldn't expect them to be links!)

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
• Correctly apply the product badges and tiers
• 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)
• PM's review (recommended, but not a blocker)
• Technical writer's review (required)
• Merge the EE-MR first, CE-MR afterwards