Documentation: Add a note to FE development process to remember to add approvers to MRs (especially UX)

What does this MR do?

As per our monthly FE/UX call this adds a reminder to the FE development process to add relevant UXers as approvers, as well as BE/FE.

Adds item to this page: https://docs.gitlab.com/ee/development/fe_guide/development_process.html#finishing-development-review

Related issues

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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