Revise extension page title for accuracy

What does this MR do?

In the 2021-11-03 Technical Writing team meeting, I was asked to revise the name of https://docs.gitlab.com/ee/user/project/repository/vscode.html (currently "GitLab Workflow VS Code extension") to "GitLab extension for VS Code". Based on https://marketplace.visualstudio.com/items?itemName=GitLab.gitlab-workflow#merge-request-reviews the extension's name is "GitLab Workflow," so I've proposed changing the name to "GitLab Workflow extension for VS Code". (cc @sselhorn)

Aside for @phikai / @viktomas - the second line in https://marketplace.visualstudio.com/items?itemName=GitLab.gitlab-workflow#merge-request-reviews is "GitLab VSCode integration" and we probably need to change to "GitLab extension for VS Code". Created gitlab-vscode-extension!371 (merged) for that change. I'll see if I can quickly figure out where this info is specified in the extension's repo, and raise a separate MR.

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