docs(integrations): fix dead Google Chat API doc links

What does this MR do?

The four doc links for the project Google Chat integration point to https://docs.gitlab.com/api/integrations.html#.... That URL now redirects to /api/integrations/, which no longer exists (it returns the docs "not found" redirect). The integrations API docs were split into project and group pages, and every other project integration link in this package already uses https://docs.gitlab.com/api/project_integrations/#....

This MR points SetProjectGoogleChatSettings, DisableProjectGoogleChat, GetProjectGoogleChatSettings and SetProjectGoogleChatOptions at the matching sections of project_integrations.

How was this checked?

• Checked the new anchors (#set-up-google-chat, #disable-google-chat, #get-google-chat-settings) against the heading ids on the live https://docs.gitlab.com/api/project_integrations/ page and against doc/api/project_integrations.md in gitlab-org/gitlab.
• Comment-only change.

This contribution was prepared with the help of an AI assistant. I reviewed the changes and take responsibility for them.