Update images and language after UI change

What does this MR do and why?

Updates https://docs.gitlab.com/ee/user/discussions/index.html to add new screenshots. The messages were updated in !70070 (merged). While I'm there, also update the icons to use the current styling, and fix a few egregious text issues.

Related to #289827 (closed)

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Related to #289827 (closed)