Update API documentation
What does this MR do and why?
To make sure the VSCode extension to send the correct data, we need to update the API documentation to reflect the correct requirements.
- Remove deprecated
v1/completions
endpoint - Update API doc for
v2/completions
Related to https://gitlab.com/gitlab-org/modelops/applied-ml/code-suggestions/ai-assist/-/issues/231
Edited by Tan Le