Moving code explanation to GA

What does this MR do?

This MR combines the Code explanation content, because it was all released via Chat, and the docs weren't fully updated.

Previously, we had three separate features listed on the main GitLab Duo page:

• Explain from IDE
• Explain from file
• Explain from MR

The last two were listed as experiments. Only the IDE was listed as GA.

All of this functionality launches Duo Chat, so this MR combines it all and makes it all GA. The top-level GitLab Duo page now lists only one feature, and it updates the LLM to match the Chat documentation, which had 3.5 Sonnet listed.

Related issues

Related to: &14261 (closed)

Author's checklist

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Reviewer's checklist

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